Symposium EQ08—Quantum Dot Optoelectronics and Low-Dimensional Semiconductor Electronics

The ability to efficiently up-convert broadband, low-intensity light would be an enabling technology for background-free biomedical imaging, volumetric 3D printing, and sensitizing silicon focal plane arrays to the short-wave infrared. Our approach uses colloidal quantum dots—size-tunable spin-mixing fluorophores—to absorb low-energy photons and sensitize the spin-triplet excitonic states of nearby conjugated molecules.^1,2,3 Once there, pairs of these long-lived excitations can combine via triplet fusion (triplet-triplet annihilation) to generate shorter-wavelength fluorescence.
To advance triplet-fusion upconversion, we are using optical spectroscopy to deepen our understanding of nanocrystal synthesis and probe the movement of energy and charge within and between organic and inorganic semiconductors.
For instance, we have uncovered that a pre-nucleation cluster intermediate has historically frustrated efforts to synthesize low-dispersity ensembles of small (d<4 nm) PbS nanocrystals, and showed that Lewis basic additives can restore one-step growth and yield markedly narrower heterogeneous linewidths in reactions that run to completion.⁴ We are expanding from this insight to build mechanistic understanding of the synthesis⁵ and surface⁶ of metal-chalcogenide nanocrystals.
We then harnessed the ultra-small (d~1.7 nm, hν_peak,abs=2.2eV) PbS quantum dots that we can now controllably produce to sensitize ‘red-to-blue’ triplet-fusion upconversion in solution.⁷ We show that the long (>µs) photoluminescence lifetimes of these particles enable max-efficiency upconversion at lower light intensities (I_th=220 mW/cm²), overcoming a mildly endothermic sensitization scheme that maximizes the anti-Stokes shift (=1.04 eV). This architecture facilitates the photo-initiated polymerization of methylmethacrylate using only long-wavelength light (λ: 637 nm); a demonstration of nanocrystal-sensitized upconversion photochemistry. Finally, from the quasi-equilibrium dynamics of triplet energy transfer, we infer that the chemical potential of photoexcited, ultra-small PbS quantum dots is surprisingly high—completing an advantageous suite of properties for upconversion photochemistry, but reinforcing questions regarding the emissive state.
References:
1. Wu, Congreve, MWBW et al. (Bulovic, Bawendi, Baldo) Nature Photon. 10:31 (2016)
2. Huang et al. (Bardeen, Tang) Nano Lett. 15:5552 (2015)
3. Mongin, et al. (Castellano) Science 351(6271):369-372 (2016
4. Green, et al. (MWBW) Chem. Mater. 32(9):4803–4094 (2020)
5. Yarur Villanueva, et al. (MWBW) ACS Nano (ASAP, 2021) 10.1021/acsnano.1c06730
6. Green, et al. (MWBW) ACS Appl. Nano. Mater. 4(6):5655–5664 (2021)
7. Imperiale, et al. (MWBW) Chemical Science. (Advanced Article, 2021) 10.1039/D1SC04330G

Charged quantum dots have been of great interest since the charged state is inevitably involved in the carrier dynamics in nanocrystals and the charge transport in nanocrystal films. Thus, it is important to make a platform to investigate the charged state in the synthesis step. Self-doping by varying the stoichiometry of the composition of a quantum dot enables tuning the carrier density and realizing the charged state of the nanocrystal in steady state. In this talk, I will present optical and electrical properties of colloidal quantum dots (AgxE (x > 2), HgE, E= S, Se, Te) with and without self-doping. The self-doping maximizes the optical selectivity by making intraband transitions available, and allows studying the the emergence of localized surface plasmon resonance of nanocrystals.

During impact ionization or inverse Auger recombination, a single hot carrier relaxes to a lower-lying state within the same band, which is accompanied by the excitation of a valence band electron across the energy gap (E_g). As this leads to generation of an additional electron-hole (e-h) pair, this process is often referred to as carrier multiplication (CM).¹ In principle, CM could improve the performance of a variety of optoelectronic, photovoltaic and photocatalytic devices.^2,3 However, practically realized CM efficiencies are still not sufficiently high to achieve an appreciable boost in device performance.
Recently, it was demonstrated that the rate of Auger-type energy transfer could be dramatically enhanced by employing spin-exchange interactions enacted by introducing magnetic impurities (Mn) into colloidal quantum dots (QDs).⁴ Extremely fast rates of spin-exchange processes allow for ‘uphill’ Auger-type energy transfer with an energy-gain rate that greatly exceeds the intraband cooling rate.^4,5 In ref. 4, this effect was exploited to realize highly efficient photoemission due to ejection of a hot electron. A highly favorable energy gain/loss-rate ratio could also enable new schemes for capturing kinetic energy of hot, unrelaxed carriers to instigate a highly efficient CM process.
In the present study, we exploit strong Mn-doping-induced enhancement in the energy gain/loss ratio for achieving low-threshold, high-yield CM due to inverse spin-exchange Auger recombination. For this purpose, we develop Mn-doped core/shell PbSe/CdSe QDs wherein the dopants exhibit strong spin-exchange coupling to both CdSe and PbSe QD components. By applying transient photoluminescence measurements, we observe a highly efficient excitation transfer from the light-harvesting CdSe shell to the Mn dopants, which is followed by two types of spin-exchange processes involving the PbSe core. In one, the difference between the energy of the excited Mn ion (hv_Mn) and the band-edge PbSe core exciton (hv_PbSe) is released in the form of a near-infrared photon whose energy (hv_SE) closely correlates with hv_Mn – hv_PbSe when hv_PbSe is tuned by changing QD dimensions. In the second process, the energy surplus given by hv_Mn – hv_PbSe relaxes via a CM-like spin-exchange process which leads to generation of two core excitons. The corresponding quantum efficiency measured at 2.6E_g is ~140%, implying that the e-h pair creation energy is less than 1.5E_g. This is near the fundamental one-bandgap limit and is also considerably smaller (a factor of >2.5) than for the reference undoped QDs. These results suggest that the use of spin-exchange interactions represents a viable approach for realizing practical CM-enhanced solar-photoconversion schemes.
1. Klimov, V. I. Ann. Rev. Condens. Matter Phys. 5, 285-316 (2014).
2. Semonin, O. E. et al. Science, 334, 1530-1533 (2011).
3. Yan, Y. et al. Nat. Energy 2, 17052 (2017).
4. Singh, R., Liu, W., Lim, J., Robel, I. & Klimov, V. I. Nat. Nanotech. 14, 1035-1041, (2019).
5. Livache, C., Kim, W.D., Jin, H., Kozlov, O.V., Fedin, I. & Klimov, V. I. et al. submitted (2021).

Solution processable colloidal quantum dot solar cells have emerged as a promising low materials cost, low embodied production energy and high-efficiency photovoltaic technology. Since 2008, small cell efficiencies have increased from 2.1% to a certified 16.6% in 2020 [1,2]. Among colloidal quantum dot solar cells, Pb chalcogenide i.e. PbS and PbSe quantum dots stood out due to their high efficiency, simple fabrication process and good stability. PbS quantum dot cells having a record efficiency of 13.8% have been demonstrated [3]. Due to the quantum confinement effect, the bandgap of the material is size dependent. This enables both optimized-bandgap single-junction cells and multi-junction tandem devices. On the other hand, a star material, metal halide perovskite quantum dots has been rising in the quantum dots family. Since the first demonstration of >10% efficiency by J. Luther’s group in 2016 [4], both efficiency and stability of perovskite quantum dot solar cells have rapidly improved with a record efficiency at 16.6% reported in 2020 [5].
The rapid development of colloidal quantum dot solar cells is largely attributed to the great research efforts leading to the advancement in materials synthesis, surface passivation, and device structure design and fabrication. Our research has been focused on surface passivation to improve device performance in terms of both efficiency and stability. Examples include the development of an effective passivation route for PbSe quantum dots using lead halide perovskite nanocrystals as Br^- or I^- sources. This approach has significantly reduced surface defect density as evidenced by enhanced PL quantum yield (PLQY) [6], leading to an efficiency of 9.2%, the highest for PbSe quantum dot cells in 2019 [7]. Surface passivation strategies for PbS quantum dots have also been developed including incorporating a thin alloyed shell on their surface leading to very high PLQY (>85%), as well as incorporating KI3 in the ligand exchange process enabling an efficiency of 12.1% [8]. In this talk, details of above novel passivation approaches developed by our research team for improving quantum dot solar cells, both Pb chalcogenides and perovskites will be presented.

Reference:
[1] Luther J. M. et al., Nano Letters 8, 3488 (2008).
[2] Hao M, et al., Nature Energy 5, 79 (2020).
[3] Sun B, et al., Joule 4, 1542 (2020).
[4] Swarnkar A, et al., Science 354, 92 (2016).
[5] Hao M, et al., Nature Energy 5, 79 (2020).
[6] Zhang Z, et al., Advanced Materials 27, 1703214 (2017).
[7] Hu L, et al., Solar RRL 2, 1800234 (2019).
[8] Hu L, eta al., Advanced Science 8, 2003138 (2021)

The potential profile and the energy level offset of core/shell heterostructured nanocrystals (h-NCs) determine the photophysical properties and the charge transport characteristics of h-NC solids. However, limited material choices for heavy metal-free III-V/II-VI h-NCs pose challenges in comprehensive control of the potential profile. Herein, we present an approach to such control by steering dipole densities at the interface of III-V/II-VI h-NCs. The controllable heterovalency at the interface is responsible for interfacial dipole densities that result in the vacuum-level shift, providing an additional knob for the control of optical and electrical characteristics of h-NCs. The synthesis of h-NCs with atomic precision allows us to correlate interfacial dipole moments with the nanocrystals’ photochemical stability and optoelectronic performance.

BiI₃ is a material that has been studied for a long time for ionizing radiation detection, but is currently emerging for new applications, such as photovoltaics, photodetectors and photoelectrohemical devices. BiI3 is non-toxic, has a bandgap of 1.67 eV, a large ionic radius, a disperse valence band, and superficial intrinsic point defects because of its 6s² lone pair of electrons, which consequently position the material among the defect-tolerant materials. Although in recent times a variety of reports of BiI₃ nanoparticles synthesis have emerged, there are some inconsistencies due to the complex chemistry of Bi³⁺ compounds. It is very difficult to obtain a pure phase of BiI₃ in solution, and the addition of ligands to control the particle size complicates the system even more. Moreover, the electron beam from the transmission electron microscopy produces nanoparticles that are easily mistaken for the particles obtained in the original synthesis, leading to erroneous conclusions. In this work, we studied the synthesis of BiI₃ nanoparticles by the hot injection and hydrothermal methods, avoiding secondary compounds such as BiOI or Bi₅O₇I₉. With the aim of achieving nanoparticles of controlled size dispersion, and to obtain suspensions appropriate for solution-processable devices, we studied aniline, pyridine, oleic acid and tryoctylphosphine as ligands. We employed bismuth neodecanoate, and iodine as source materials, and 1-octadecene as solvent. We could determine a set of synthesis conditions to obtain pure BiI₃ nanoparticles of about 110 nm in size, capped with aniline or tryoctylphosphine. Moreover, we achieved stable suspensions in chloroform, and prepared thin films by drop-casting onto gold-covered glasses. Finally, the metallic cathode was evaporated onto the film surface with a mask. Optical and morphological properties were investigated through SEM and absorption spectroscopy. The development of BiI₃ nanoparticles capable of forming stable suspensions is promising, due to the interesting optical properties that they have, in addition to fewer production costs for the potential devices.

The remarkable tunability of quantum dot (QD) superlattices (SLs) make them versatile candidates for advancing optoelectronic devices as well as investigating exotic electronic phenomena. Due to the complicated interactions at work during superlattice formation and oriented attachment, however, defects and disorder have so far prohibited the realization of electron delocalization and tunable miniband formation. Controlled heating offers a promising route to improve these electronic properties through defect relaxation. Here, we employ in situ MEMS heating in combination with atomic-resolution scanning transmission electron microscopy (STEM) and image analysis to track the evolution of epitaxial connections in square PbSe QD SLs over ~100 nm fields of view. We identify four distinct strain defect types – tensile, shear, bending and twisting – and visualize them using real space, local strain and out-of-plane orientation mapping techniques. We find that upon heating tensile and shear defects are fully relaxed, and twisting defects are reduced, but bending defects remain. We attribute the persistence of bending defects in part to constraints imposed by connections with the four neighboring QDs. To test this hypothesis, we also consider connected QD wires, a possible precursor to square SLs, wherein QDs are constrained by connections in only one dimension. These results improve our understanding of the energetics of defects present in epitaxial connections and support a route toward improved electron delocalization in connected QD SLs.

Quantum dot LEDs (QLEDs) are promising candidates for next-generation self-emitting displays due to their high color gamut and narrow emission spectrum. Quantum dots cannot be deposited by vacuum deposition, but QLEDs can be fabricated using solution processes such as inkjet printing. Although CdSe quantum dots are being studied most actively, displays using Cd-based quantum dots are regulated by the European Restriction of Hazardous Substances Directive (RoHS). Among Cd-free quantum dots, InP shows the best performance¹ and has already been commercialized in red and green. For blue, ternary ZnSeTe quantum dots are the most promising materials.²
We synthesized blue ZnSeTe quantum dots for use in QLED devices. As the Te/Se ratio of the core increased, the PL wavelength and the emission peak width increased, showing the same trend as previously reported.³
The EL quantum efficiency did not change significantly while the electroluminescence lifetime increased when the Te/Se ratio was increased. To achieve high EL quantum efficiencies, an optimal thickness was required for both the ZnSe inner shell and the ZnS outer shell. Through optimization, blue quantum dots of the emission peak wavelength around 460 nm with excellent electroluminescent properties can be synthesized. The surface of these quantum dots was further treated then an ink formulation was prepared using an appropriate mixed solvent which slows down the drying rate at the nozzle, stabilizing the drop placement position of ejected droplets, ensuring wetting in the pixel and causing Marangoni flow to increase the flatness of the pixel. 6.95-inch and other size full color QLED panels with top emission architecture were made by inkjet process using this ZnTeSe-blue and InP- red and green quantum dot inks.

Quantum dot light-emitting diodes (QLEDs) are one of the most promising candidates for next generation display due to their advantages such as high photoluminescence quantum yield (PL QY), narrow bandwidth, and facile color tunability of quantum dot (QD) materials. Although the performance of QLEDs have been improved significantly, they are exclusively optimized for the display brightness since those QLEDs are usually vulnerable to high electrical or thermal stress. For QLEDs to exhibit higher luminance (L), therefore, it is important to resolve those degradation issues at an intense operating condition. Here, we demonstrate ultra-bright QLEDs by adopting multi-lateral approaches—from the synthesis of QD materials to the elaborate engineering of the device structure. First, the non-radiative Auger recombination under high current densities (J) is effectively suppressed by tailoring the core/shell interfaces of QDs. Second, the top-emission device structure is optimized based on optical simulation to enhance the light outcoupling. Finally, the thermally conductive Si substrate is used to dissipate thermal energies at high J. Combining these approaches, our red top-emitting QLEDs exhibit unprecedently high maximum L and the current efficiency, which exceed 3,000,000 cd m⁻² and 75 cd A⁻¹ respectively. We believe that our breakthrough can make QLEDs explore to other diversified and specialized fields, such as lighting source, laser diode, or photo-medical therapy.

Due to excellent electronic and optical properties, indium phosphide (InP) has garnered considerable attention in optoelectronic applications. Based on the direct band gap with large exciton Bohr radius, the nanocrystalline form of InP can cover the emission wavelength from blue to red and up to NIR. The non-toxic composition also allows the commercial use of InP NCs as a light source for display, bio-imaging, and solid-state lighting. However, without epitaxially grown shells, the InP NCs are not enough to exhibit high luminescent properties because of their surface trap sites caused by dangling bonds and oxidative defects. Recent studies have revealed that unavoidable surface oxidation of the InP cores can occur even in the inert reaction atmosphere. Chemical modification of InP core surface is, thus, required prior to the beginning of the shell formation since the defective species on InP cores may lower photoluminescence yield and interrupt conformal shell growth. Among various approaches, etching the surface of InP NCs by adding a Lewis acid such as hydrofluoric acid has been considered as an effective way to eliminate surface oxides. Such surface modification using Lewis acidic ligands increases photoluminescence quantum yield (PLQY) in semiconductor NCs. However, the effect of the ions or molecules involved in the etchants on shell growth is still veiled. In the shelling process, various precursors are employed together and may inadvertently react with each other at high temperature. The effect of surface oxide etchants should be considered not only for preparing the neat surface of InP cores, but also for developing sophisticated shell growth.
Here, we investigate the combinational effect of chemical precursors on the PL improvement of InP cores as well as on the formation of core/shell structures. Based on a previous study that metal carboxylate can etch the surface of semiconductor NCs, we employ liquid coordination complexes (LCCs) that can passivate the dangling bonds of InP cores. LCCs are the eutectic mixture of the metal halide and organic donors such as amines and phosphines. The coordination compounds are ionic liquid-state Lewis acids, which can be used as an effective Z-type ligand of NCs, especially when zinc carboxylates are being in place together. The evolution of optical properties of the InP cores reveals that the combination of LCC and zinc carboxylates can remove the surface trap sites and passivate the neat InP surface, increasing PLQY of InP cores to 50%. The in-situ defect-removal process using the precursor combination is confirmed by surface analysis including X-ray photoelectron spectroscopy and solid-state ¹H-³¹P NMR. The surface cleaning and brightening effects of LCC on InP cores benefit even after the formation of shell structure. The LCC can, in addition, be converted to a good source for shell growth once the reaction mixture reaches shell growth temperature. As a result, the resultant InP/ZnSeS/ZnS NCs exhibit the PLQY over 95%, and the proof-of-concept electroluminescence devices incorporating the NCs show high current efficiency above 30 cd/A and maximum luminance of 11000 cd/m² for green emission.

Colloidal quantum dots (QDs) have been extensively studied for applications in lasers, light-emitting diodes, field-effect transistors, solar concentrators, and solar cells. Still, there is a need for high-quality QDs emitting in the near-infrared (NIR) and exhibiting fast emission rates, 'non-blinking' intensity trajectories, high spectral stability, and wide-range spectral tunability. Such dots would be of great practical utility in optical communications, quantum networks and biomedical diagnostics. Here we present recent work on the synthesis, and electronic and optical characterization of a new type of NIR emitters that are derived from well-known visible-light emitting CdSe/CdS QD heterostructures (1). Via incorporation of an atomically-defined HgS interlayer at the CdSe/CdS interface we are able to shift the QD emission into the NIR and achieve continuous tunability from ~700 to ~1400 nm. The NIR emission is realized thanks to the spherical quantum well formed by the narrow-gap HgS interlayer, which traps an injected electron and whereby down-converts the emission energy.

To grow the HgS interlayer, we replace the original organic ligands on the initial CdSe cores with S^2- species in the presence of oleylamine (OLA). This S^2- surface plays the role of a 'wetting' layer, facilitating the highly uniform, spherically symmetric growth of HgS. To complete a full HgS monolayer, the S^2-/OLA-capped CdSe cores are reacted with mercury acetate dissolved in OLA using the 'exact' amount of mercury precursor determined by the number of Hg²⁺ ions required for complete coverage of the QD surfaces, analogous to the approach used with the successive ionic layer adsorption and reaction (SILAR) technique. By systematically repeating the deposition of complete anionic and cationic layers, we are able to grow an HgS interlayer with monolayer (ML) control with HgS thickness varied from 0 up to 6 MLs. Finally, CdS protective shells are grown on top of the CdSe/HgS QDs using c-ALD method, which serves to eliminate residual interfacial defects resulting in quantum yields in excess of 70%.

To confirm the fabrication of CdSe/HgS/CdS QDs with an atomically-defined HgS layer, we correlate our steady-state absorption/PL data with rigorous size analyses using high-resolution TEM measurements, and further benchmark them against electronic states calculations based on the effective-mass approximation (EMA). We find that the average QD size increase measured by TEM is consistent with the growth of atomically-defined monolayers, and excellent agreement is found between optical characterization and the EMA calculations for a wide variety of CdSe core sizes, and HgS interlayer thicknesses.

Finally, we analyze the optical properties of CdSe/HgS/CdS QDs. With the growth of a thicker (4-6 ML) CdS shell, near single-exponential lifetimes are achieved with radiative time constants as short as 40 ns. This is comparable to the lifetimes achieved with NIR-emitting InAs/CdZnS QDs, and orders of magnitude shorter than the lifetimes of traditional PbSe(S)-based NIR emitters. Further, QDs with a thick CdS shell show virtually non-blinking emission at the single QD level, with single QD 'ON' fractions averaging ~90%. Lastly, we evaluate the single-photon purity of these QDs from the second-order intensity correlation function g²(τ), where we find two-photon coincidence counts (i.e, g²(0)) as low as 0.04, indicating nearly perfect photon anti-bunching. Such high-quality single photon sources, with characteristics akin to their CdSe/CdS counterparts but emitting in the NIR range, will be of great interest for applications in quantum telecommunications and biomedical imaging.

1. Sayevich, V., Robinson, Z.L., Kim, Y. et al. Highly versatile near-infrared emitters based on an atomically defined HgS interlayer embedded into a CdSe/CdS quantum dot. Nat. Nanotechnol. 16, 673–679 (2021). https://doi.org/10.1038/s41565-021-00871-x

The self-organization of colloidal nanoparticles into programmed suprastructures has been of great interest in a wide range of fields like nano, colloid, and polymer sciences owing to the novel multifunctional or collective physicochemical properties exhibited from the organized suprastructures [1, 2]. In addition, they are rendered as new promising materials for a wide range of application fields with their excellent colloidal stability. However, despite the recent progress in their fundamental understanding and practical applications, the self-organization of all-inorganic nanoparticles remains unexplored. The interparticle interactions among colloidal nanoparticles are usually governed by organic soft stabilizers owing to the easy introduction and controllability. However, the interactions between long-chain organic ligands are quite complex because of their multiplicities and related interdependencies [3].
Herein, we present the controlled organization of oppositely charged all-inorganic nanoparticles through the electrostatic interaction and the colloidal behaviors of organized suprastructures. Three different phases of patchy, patchy bridged, and fully coated particles are classified depending on the charge states and colloidal behavior of the organized suprastructures. The phase behavior was investigated with the factors of number ratio and size ratio of oppositely charged nanoparticles. Furthermore, the phase diagram is constructed with the concentration of oppositely charged nanoparticles. Especially, the fully coated particles exhibit replicable colloidal stability through the action of nanoparticles as surface stabilizers to induce the overcharged surface state; thus, the concept of “nano-ligands” is proposed. Further experiments demonstrate that a wide range of material combinations, including semiconducting, metallic, and oxide nanoparticles is applicable for the concept of nano-ligands. The currently developed approach will enable the chemical designing of self-organized nanostructures [4].
References
E. V. Shevchenko, M. Ringler, A. Schwemer, D. V. Talapin, T. A. Klar, A. L. Rogach, J. Feldmann and A. P. Alivisatos, J. Am. Chem. Soc. 130, 3274 (2008).
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C. A. Batista, R. G. Larson and N. A. Kotov, Science 350, 1242477 (2015).
D.H. Gu, J. Lee, H. W. Ban, G. Lee, M. Song, W. Choi, S. Baek, H. Jeong, S. Y. Lee, Y. Choi, J. Park, Y. I. Park and J. S. Son, Chem. Mater. 32, 19, 8662 (2020).

Metal nanoparticles have been studied for anisotropic nanoparticles via colloidal synthesis. Silver nanoparticles, for example, have been synthesized as 0D-3D shapes such as nanospheres, nanowires, nanoplates, and nanocubes. However, the synthesis of different shapes of colloidal semiconductor nanoparticles has not been studied as many as metal nanoparticles. Here, we present a new method to synthesize anisotropic colloidal semiconductor nanoparticles using metal nanoparticles as a template. Shaped metal nanoparticles can be utilized to synthesize semiconductor nanoparticles via chemical reactions. For example, colloidal metal nanoparticle reacts with chalcogenide to form metal chalcogenide semiconductor nanoparticles without deformation. The metal can be displaced with different metals for different semiconductor nanoparticles. Indeed, the new method enables to synthesize semiconductor nanoparticles with different compositions from a single metal nanoparticle. Furthermore, this method enables to synthesize anisotropic semiconductor nanoparticles including the shapes that cannot be achieved from colloidal synthesis.

The optoelectronic properties of semiconductor nanocrystals, or quantum dots (QD), are dependent on their surface chemistry which is often altered through ligand exchange reactions or growth of inorganic shells after their initial synthesis. Analysis and developed understanding of their surface modification processes are important to finely tune QD properties to their intended application, such as biosensors, solar cells, light-emitting diodes, and (photo)electrocatalysts. To that end, we employed isothermal titration calorimetry (ITC) to investigate the ligand exchange of oleate for dodecylphosphonic acid (DDPA) on CdSe QDs. The thermograms revealed an initial exothermic response follow by a delayed endothermic response upon injection of DDPA into the QD solution. We derived a new model to analyze the ITC isotherms that accounts for the two-step process observed and represents the physical processes occurring. Enthalpy of reaction (ΔH), equilibrium constant (K), and stoichiometry of the reaction (n, or number of binding ligands per QD) were extracted from the ITC analysis, which were then used to determine the entropy (ΔS) and Gibbs free energy (ΔG) for the processes. The initial exothermic response is attributed to the ligand exchange of oleate for dodecylphosphonic acid, and we used quantitative nuclear magnetic resonance spectroscopy to confirm the stoichiometry of the reaction determined by ITC. Interestingly, the endothermic response disappears when the exothermic ligand exchange is only ~76% complete. X-ray diffraction (XRD) revealed a slight increase in strain in the nanocrystal lattice, and Raman spectroscopy revealed a shift in signals corresponding to the surface optical and transverse optical phonons of the DDPA-treated CdSe nanocrystals. The XRD and Raman results indicate that ligand-driven surface atom rearrangement occurs on the QDs and is potentially responsible for the delayed endothermic response observed in ITC in this ligand exchange reaction. This assignment is the first direct experimental observation of ligand-induced surface atom rearrangement on colloidal nanomaterials. The depth of fundamental information afforded by this technique is essential to advance the ability to finely tune QD properties to their applications and will ultimately lead to enhanced efficiency of devices utilizing these low-dimensional semiconductor nanomaterials.

Proper control of the luminescence from silicon quantum dots (SiQDs) would allow their commercial use in technologies ranging from artificial lighting to optical computing applications [1,2]. Imbibe SiQDs in a silicon nitride (Si3N4) thin film is a good option for technology transfer because Si3N4 has a good concentration of charge carriers and a moderate band gap that enables low turn-on voltages in operational devices [3].
Multiple studies have shown that the quantum confinement effect is largely responsible for the absorption-emission characteristics of SiQDs embedded in a Si3N4 matrix [4–6]. In other words, it is possible to tailor (within certain limits) absorption, emission, or both by changing the SiQDs average size and population density in those systems [7]. This design capability makes these systems very attractive because they can be used in electroluminescent or optoelectronic devices, sensors, and solar cells [8,9].
In this work, we present a comprehensive analysis based primarily on High-Resolution Transmission Electron Microscopy (HRTEM) to report the SiQDs characteristics of five embedded systems. Each system was grown by RPECVD on the surface of a single-crystalline n-type (100) silicon wafer, fused silica, highly oriented pyrolytic graphite (HOPG), muscovite mica, and single-crystalline potassium chloride. We found that the growths on single crystalline silicon and fused silica have SiQDs with better luminescence characteristics, i.e., reduced average size and high population density. These two substrates are followed by the HOPG, which also exhibits acceptable SiQDs properties. Meanwhile, muscovite mica and potassium chloride show the worst SiQDs characteristics, i.e., bigger average sizes and populations without normal distributions. We conclude that chemical affinity between the substrate and the precursor gases is the most critical parameter to achieve SiQDs formation and reproducibility.
References
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[8] A.L. Muñoz-Rosas, A. Rodríguez-Gómez, J.A. Arenas-Alatorre, J.C. Alonso-Huitrón, Photoluminescence enhancement from silicon quantum dots located in the vicinity of a monolayer of gold nanoparticles, RSC Adv. 5 (2015) 92923–92931.
[9] A. Muñoz-Rosas, A. Rodríguez-Gómez, J. Alonso-Huitrón, Enhanced Electroluminescence from Silicon Quantum Dots Embedded in Silicon Nitride Thin Films Coupled with Gold Nanoparticles in Light Emitting Devices, Nanomaterials. 8 (2018) 182.

Semiconductor nanocrystal quantum dots possess unique optoelectronic properties dependent on their size, shape, and elemental composition. As such, determining their chemical composition is essential to understanding their structure-property relationships to enhance current and/or enable new applications of these low-dimensional semiconductor nanomaterials. Current methods used to measure their composition employ large, expensive instrumentation such as energy dispersive x-ray spectroscopy or inductively coupled plasma optical emission spectroscopy (ICP-OES), which is destructive to the sample. While large and expensive laboratory-scale X-ray fluorescence spectrometers have also been employed to investigate the elemental composition of some nanomaterials, cost-effective portable X-ray fluorescence (pXRF) spectrometers have not yet been demonstrated to have the necessary sensitivity to accurately determine the chemical composition of nanomaterials. pXRF spectroscopy is a handheld non-destructive instrumental technique operated on a simple benchtop under ambient conditions with minimal sample preparation. We experimentally measured the chemical composition of multiple sizes of colloidal solutions of CdSe quantum dots with pXRF and determined the elemental Cd/Se ratio of 3.3 nm and 2.3 nm CdSe nanocrystals to be 1.37±0.08 and 1.5±0.1, respectively. These results agree with previous literature, and we further validated the results with ICP-OES. We also determined figures of merit for the pXRF spectrometer and found limits of detection for Cd and Se to be 56 ppm and 54 ppm, respectively. Recently, we have extended these studies to CsPbBr₃ perovskite quantum dots and demonstrated that pXRF can be applied to different colloidal systems to determine elemental composition. This work demonstrates that colloidal nanomaterials of various compositions can be investigated at the atomic level with portable, cost-effective, and non-destructive chemical instrumentation. pXRF is a cheaper alternative to the ‘standard’ analytical techniques currently employed to interrogate elemental composition of colloidal nanomaterials and our work with this affordable technique allows for nanomaterials research to be accessible to research laboratories and institutions that may have limited resources for facilities and instrumentation.

Lead-chalcongenide quantum-dots have been widely studied because they show the property of multiple-exciton generation for high-energy photons. A functional device that incorporates multiple-exciton generation would have to be composed of an organized lattice of quantum-dots that are epitaxially fused at the interfaces. A perfect lattice might be expected to achieve mobility of 50 cm^2/Vs but measured is much lower. To investigate the structural origin of lower charge mobility we use high-resolution electron tomography to achieve near atomistic resolution imaging. We show atomic orientation variation for 1000s of QDs in several different ordered superlattice samples. Unlike previous studies, we study the position and alignment changes both laterally and vertically through samples with as many as 13 layers of QD particles. In addition to studying the variation in position and orientation across the sample, we also measure the thickness of epitaxial bridges between neighboring particles. Comparing the superlattice positions of the particles with the distribution of atomic lattice orientations shows that superlattice disorder is driven by the difference between the cubic structure of the atomic lattice vs the triclinic superlattice. Electron tomography proves to be a powerful tool for investigating quantum-dot superlattice samples.

Recently Localized Surface Plasmon Resonance(LSPR) in a semiconductor has attracted a lot of attention due to its unique property that combines LSPR and the quantized energy, which are the characteristics of metal and semiconductor, respectively. Here, we present the appearance of LSPR in self-doped Ag₂Se nanocrystals mixed with the intraband excitonic character. By fitting the experimental data to the k.p model, it was observed that absorption peaks deviated as the size of nanocrystals increased while they agreed well with the model including the surface plasmon resonance term, which indicates that the LSPR property gradually mixed with the excitonic transition. Additionally, the intensity of photoluminescence attenuated with increasing the size, demonstrating the weakening of excitonic transition and emergence of LSPR property. Interestingly, the peak was observed to split as the LSPR character increases, which simultaneously occurs with the crystal phase transformation from cubic to tetragonal. By applying the electrochemical potential to the nanocrystals, it was revealed that both split peaks share the same state, 1S_e, which indicated that the peak splitting results from the nondegenerate 1P_e state. The observed transformation of optical property in self-doped Ag₂Se nanocrystals paves the way for the new colloidal quantum to plasmon hybrid optoelectronics and provides an opportunity to investigate the correlation between the crystal phase and carrier density.

Here, we study how the incorporation of Cd into mercury telluride (HgTe) nanocrystal quantum dots affects their electrical transport properties. The radial concentration distribution of Cd in HgTe nanocrystals was controlled synthetically by adding Cd(oleate)₂ into a heated solution of HgTe nanocrystals and excess oleylamine (OAm). OAm serves as a capping ligand and in the ligand-rich environment, the shape of the nanocrystals changes from tripods to spheres and the related surface reconstruction facilitates dopant incorporation. Addition of the Cd reactant at temperatures greater than 200 ^oC produces uniformly doped mercury cadmium telluride (MCT) with a controlled doping concentration. Shells enriched in Cd were deposited with a controlled thickness by adding both Cd and Te reactants at a lower temperature of 175 ^oC. The nanocrystal films in field effect transistor devices exhibit p-type behavior. STEM-EDS maps of the nanocrystals showed that there is a radial distribution of Cd in the nanocrystals with the presence of non-stoichiometric cation vacancies. The carrier mobility in the films of HgTe nanocrystals with Cd-enriched surfaces is 0.03 cm² V^-1s^-1, which is three times higher than the MCT nanocrystals with uniform Cd composition. The higher mobility is attributed to a hole carrier pathway by valence band alignment between HgTe and CdTe and leads to significantly improved, faster photoresponse under solar and IR illumination.

Semi-automated and fully automated platforms, in combination with data-science principles and artificial intelligence, have become an emerging paradigm for accelerated materials discovery[1]. The combination of high-throughput experimentation and minimal human interactions with the system have allowed faster material synthesis, characterization, and analysis. However, many initiatives of materials acceleration platforms (MAPs) are still too costly to be implemented. In this context, open hardware principles (i. e. distribution of design files, schematics, drawings and source codes for hardware projects) have made the use of laboratory automation more accessible and more easily implemented for a variety of applications. Here we present the use of a repurposed open-hardware and software 3D printing platform, Jubilee (https://github.com/machineagency/jubilee), as a plate handling lab automation device for sonochemical applications. The system, called sonication station, is an open-source flexible automation platform with a sonication horn attachment which allows for an array of automated experiments through the easy modification of several sonication parameters. Building upon the sonochemical synthesis of quantum dots and magic-sized clusters (MSCs) [2], a fully automated protocol was developed to demonstrate the synthesis of CdSe nanocrystals using sonochemistry and different combinations of sample conditions, including precursor ratio and ligand composition. The sonication station was used in combination with a liquid handling robot OT2 (Opentrons) for the sample preparation, to further automate the workflow for nanocrystal synthesis. The system allows to investigate and process upwards of 96 different samples, with a total sample volume of as little as 0.5 mL. The proposed workflow uses low-budget systems (< $10k for both robotic platforms) and the reduced volume allows for a more cost-efficient experimentation, increasing the accessibility of this MAP. Thanks to the high-throughput capabilities of the automated sonication platform, the ease in scalability of the system, and the modularity of the protocol, the overall workflow is adaptable to a variety of studies, including other nanocrystals design spaces, emulsions and re-dispersions.


[1] “Materials Acceleration Platforms: On the way to autonomous experimentation”. Flores-Leonar, M. M., Mejía-Mendoza, L. M., Aguilar-Granda, A., Sanchez-Lengeling, B., Tribukait, H., Amador-Bedolla, C., & Aspuru-Guzik, A. , Current Opinion in Green and Sustainable Chemistry, 25, 100370 (2020)

[2] “On-Demand Sonochemical Synthesis of Ultrasmall and Magic-Size CdSe Quantum Dots in Single-Phase and Emulsion Systems”, R Kastilani, B. Bishop, V. Holmberg, LD Pozzo, Langmuir, 35 (50), 16583 (2019)

The graphene quantum dots (GQDs), a zero-dimensional graphene quantum structure, have triggered an intense research worldwide. GQDs possess unique optical, chemical and physical properties as compared to conventional quantum dots (QDs), such as low toxicity, biocompatibility, optical stability, chemical inertness, high photostability and good water-solubility and therefore hold great application potential in biomedical, optoelectronics and energy storage devices. The doping of GQDs with heteroatoms is one of the most effective ways to tune their photoluminescence emission and to increase quantum yield. In this study, we developed a novel approach to synthesize high-quality Nitrogen-doped graphene quantum dots (N-GQDs) with high quantum yield, via irradiation of s-triazene in a solution with benzene by using pulsed laser. The TEM, HRTEM, XPS, XRD, Raman spectroscopy and FTIR were carried out to observe the morphology, size distribution, crystalline structure and to prove successful doping of GQDs with nitrogen atoms. To observe optical properties of as synthesized N-GQDs, the UV-vis and Photoluminescence measurements were carried out. The as-synthesized NGQDs exhibit high quality crystalline structure of graphene with an average size of about 3.7 nm. A high quantum yield was exhibited by the obtained N-GQDs as compare to the pristine GQDs. The obtained N-GQDs with oxygen-rich functional groups exhibit a strong emission and excellent upconversion PL properties. These outcomes result in an ample opportunity for the biomedical and optoelectronic applications.

Field emission is a quantum mechanical phenomenon where electrons tunnel from the cathode to the anode through vacuum upon an applied electric field. So far field emission properties of two-dimensional (graphene) and one-dimensional (CNT) carbon nanostructures have been extensively studied but, to the best of our knowledge, no study is available on FE properties of zero-dimensional carbon nanostructures (GQDs). In the current study, we report the field emission properties of GQDs and N-GQDs by measuring turn-on field (E_to) (found to be 7.9 V/μm and 13.1 V/μm, respectively) and field enhancement factor β (found to be 258, 176 for low-field region and 1427, 2511 for high-field region respectively). The results show that nitrogen doping improved the field emission properties of GQDs by reducing turn-on field from 13.1 V/μm (GQDs) to 7.9 V/μm (N-GQDs) and enhancing the field enhancement factor β from 1427 (GQDs) to 2511 (N-GQDs). The value β for N-GQDs was found to be much higher than that of pristine graphene (2166). Since field emission properties depend on several factors such as nanostructure geometry, work function, morphology, substrate conductivity and presence of defects etc. so there is a lot of room for research work in the case of FE properties of GQDs and to find the optimal set of conditions by changing above mentioned factors to further improve the field emission properties of GQDs and make them a potential substitute for graphene, CNT and ZnO nanoparticles. Secondly, field emission of graphene both undoped and decorated with different nanostructures have been extensively studied. There is a report in which FE properties of graphene was improved by decorating with ZnO quantum dots. So, it will be an interesting idea to measure FE properties of graphene by decorating with GQDs and find the conditions to improve the FE to a level better than the previously observed results.

Germanium quantum dots (Ge QDs) are thought to be potential candidates for future semiconductor applications, such as solar cells, photodetectors, and transistors. Here, we investigate the optoelectronic characteristics for a new synthesis technique for colloidal monodisperse Ge QDs. Specifically, we have fabricated neat films and solar cell devices using a spectrum of Ge QDs, varied by size and synthesis method, and have characterized their optical and optoelectronic properties to assess their promise as photovoltaics and photodetectors.

Graphene and highly luminescent graphene quantum dots (GQDs) have been widely used in optoelectronic devices as a photoactive material. In this study, we report the preparation of a GQDs/Graphene heterostructure in order to investigate the effect of GQDs on photoactive response of graphene. Using UV–vis absorption and photoluminescence (PL) spectra, the optical properties of graphene and the GQDs/graphene heterostructure were measured and compared. Moreover, to investigate their electronic and charge transfer properties, we fabricated field-effect transistors (FET) on pristine graphene and GQDs/graphene heterostructure thin films and investigated their photoactive electrical properties. Under illumination, both pristine and GQDs/graphene FET showed an increase in current and carrier mobility. The increased current and carrier mobility of GQDs/graphene FET is due to the presence of a large number of photoexcited charge carriers. The current and carrier mobility in the GQDs/graphene heterostructure FET were also lower than those in the pristine graphene FET. This is explained by GQDs' n-type doping effect on graphene, which reduces the accumulation of holes in the active p-channel near the insulating layer and causes charge to be transferred from the GQDs to the graphene. As a result, we discovered a charge transfer effect in the GQDs/graphene heterostructure, which could be used in optoelectronic devices.

Gallium Phosphide is an indirect bandgap material with a zinc blende structure, and thus it is difficult to achieve high photoluminescence (PL) efficiency. However, it has been predicted that the indirect bandgap characteristics can be alleviated through the quantum confinement effect. In particular, there was a theoretical prediction that GaP nanoparticles turn into a direct bandgap material when the size of 2 nm or less. However, the difficulty of synthesizing GaP NPs smaller than 2 nm has hindered its experimental confirmation.
In this study, the strong PL characteristics of GaP were achieved through a unique structure, that is ZnS/GaP reverse type I quantum dot. In detail, we synthesized a GaP colloidal quantum well, in which ZnS nanocrystal is used as a seed and GaP is stacked on the ZnS surface with a very thin thickness of 2 nm or less through. This structure provides distinct advantages in terms of energy level and synthesis. First, nanocrystal ZnS seed has a larger bandgap than GaP, and therefore conduction band minimum (CBM) and valence band maximum (VBM) are located higher and lower than GaP, respectively. As a result, electrons and holes are confined to the GaP quantum well and do not interfere with exciton recombination in GaP. In addition, since the lattice parameter difference is very small as 0.05A (5.40A for ZnS and 5.45A for GaP), GaP can be uniformly stacked on the ZnS surface. Because of these advantages, ZnS/GaP achieved a high PL quantum yield of 77% even after several purification processes. Furthermore, the GaP quantum well showed a change in the PL peak from 398 nm to 412 nm depending on the synthesis temperature and time change. This work goes beyond confirming the strong PL characteristics due to the direct bandgap of GaP and suggests a new possibility that GaP can also be used as a non-Cd-based quantum dot material alternative to InP or ZnSe.

Colloidal quantum dots (QDs) are semiconductor nanocrystals that can tune the properties by controlling size and surfaces. Although QDs have been explored for 30 years, the study for understanding and controlling their surfaces is still highly required, especially, in III-V QD. Here, we demonstrate the controllable colloidal growth in InP QD synthesis using late-intermediates. Our research shows 1) observation of intermediates, 2) isolation of the intermediate, 3) define their crystal structure and surfaces, 4) use of them as a growth platform through the understanding of their surface energy and activation energy barrier of specific growth direction. As a result, we could intentionally control the growth of tetrapod-intermediates into tetrapods having longer arm length tetrapods, or tetrahedrons. Moreover, the InP tetrapods have a single-crystalline zinc-blende crystal structure in both center and arm. It is distinguished from conventional heterostructure CdSe, CdS tetrapod nanocrystals that have zinc-blende, and wurtzite crystal structures for a center and arms, respectively. We expect our single-crystalline InP nano-tetrapod will be a new suitable tool for the study of geometry effects in quantum dots properties or exciton behavior control.

In the pandemic era, the Internet of Things (IoT) technology has grown as a critical component of the 4th industrial revolution. Various electronic devices are connected through internet networks collecting tremendous amounts of data. A substantial portion of data is derived from smart sensors that detect diverse data and convert it into electric signals. As a result, the development of high-performance indoor air quality monitoring sensors has become more critical than ever. Among diverse harmful gases in our environment, nitrogen dioxide (NO₂) is one of the most toxic gases in our daily life, which induces severe respiratory diseases such as asthma. To detect an extremely low concentration of NO₂ in exhaled breath, gas sensors with high sensitivity, fast response, low electrical power consumption, and simple operation are highly required. Conventional chemoresistive gas sensors were based on metal oxides operable at an elevated temperature over 200 ^oC. An external heating system is necessary for high temperature operation, which increases power consumption and fabrication cost. Therefore, room temperature operation with low power consumption and high sensing performance has become vital in chemoersistive gas sensors. Among various activation strategies, light activation is one of the most promising strategy which can be conducted by illuminating light source on the sensing material. Light emitting diode (LED) is the most widely used light source in light activation due to low power consumption, low price, and environmentally friendly properties.
Herein, we propose the room temperature NO₂ sensing of SnS₂ nanoflowers (NFs) enabled by visible light activation. LED light with different wavelength was illuminated on the sensing area with controlled intensity. SnS₂ was selected as a sensing material which exhibit prospective features for chemoresistive gas sensor, including semiconducting band gap, high electronegativity, and high surface area. However, SnS₂ based chemoresistive gas sensors are mostly operated at elevated temperature. Thus, fabrication of SnS₂ nanostructure which have high light absorbing properties is suggested. SnS₂ NFs were successfully prepared by solvothermal synthesis with abundant edge sites. The high absorbance of edge sites in the visible light region triggered the generation of charge carriers resulting in decreased resistance and enhanced gas sensing characteristics. Even under red and green light with low photon energy, the room temperature NO₂ sensing performance was improved. The highest NO₂ sensing performance was accomplished under blue light, including the highest response, excellent selectivity towards NO₂, and an extremely low detection limit. Moreover, the sensor exhibited reliable gas sensing performance in humid conditions and maintained its properties after long-term relaxation. Taking advantage of surface properties, optical properties, and gas sensing properties, a light-activated SnS₂ NFs based gas sensor is anticipated to further develop SnS₂ nanostructures for use in an electronic nose.

Ge-Sn alloys are interesting semiconductor materials that can exhibit a direct gap transition under appropriate composition and strain conditions. Therefore, they have been investigated as group-IV optoelectronic elements that are chemically compatible with silicon for on-chip integration in applications including mid-IR detection and chip-to-chip communications. However, the low temperatures used to grow metastable Ge-Sn alloys result in a high concentration of grown-in vacancy-related defects which are reported to act as p-type dopants and may also introduce trap states near mid-gap that promote non-radiative carrier recombination. To characterize these phenomena, we have synthesized Ge/Ge-Sn core shell/nanowire structures with well-controlled composition and strain state, and correlated extended x-ray absorption fine structure spectroscopy (EXAFS) with single wire electrical measurements. Given its sensitivity to coordination number, EXAFS can provide a direct measurement of the vacancy concentration, especially neighboring the target species. Complementary four-point probe measurements can evaluate the resistivity and estimate the field-effect mobility of carriers and carrier concentration in the core/shell nanowires. Efforts toward three-contact Hall measurements on the nanowires to quantitatively probe trends in the majority carrier type and concentration will also be described. These EXAFS and electrical measurements have been performed across samples of varying Sn compositions in the 8–12 at% range and post-growth annealing at temperatures up to 500 °C.

Long-distance two-qubit coupling, mediated by a superconducting resonator, is a leading paradigm for performing entangling operations in a quantum computer based on spins in semiconducting materials. Here, we demonstrate a novel, controllable spin-photon coupling based on a longitudinal interaction between a spin qubit and a resonator. We show that coupling a singlet-triplet qubit to a high-impedance superconducting resonator can produce the desired longitudinal coupling when the qubit is driven near the resonator’s fre- quency. We measure the energy splitting of the qubit as a function of the drive amplitude and frequency of a microwave signal applied near the resonator antinode, revealing pronounced effects close to the resonator frequency due to longitudinal coupling. By tuning the amplitude of the drive, we reach a regime with longi- tudinal coupling exceeding 1 MHz. This demonstrates a new mechanism for qubit-resonator coupling, and represents a stepping stone towards producing high-fidelity two-qubit gates mediated by a superconducting resonator.

Two dimensional (2D) semiconductors or quantum wells grown by epitaxy have been intensely studied in the 1990’s. While these investigations showcased the huge potential of 2D excitons for opto-electronics, the need for cryogenic temperatures hampered many applications. In recent years, several new quantum well systems characterized by a low-screening dielectric environment, such as monolayers of CdSe or CsPbBr₃ have been developed. These materials feature a strongly enhanced exciton binding energy and sustain stable 2D excitons at room temperature. However, it remains unclear to what extend exciton-related transitions determine the opto-electronic properties – most notably stimulated emission – of these low-screening quantum wells, and how we can understand such processes for 2D excitons with in-plane mobility.

In this study, we address this issue taking high quality CdSe colloidal quantum wells (CQWs) as a model system for which we analyse stimulated emission using quantitative and polarization resolved pump-probe and luminescence spectroscopy. We give experimental proof that mobile excitons can form stable excitonic molecules – a two-exciton complex - in such quantum wells, even at room temperature. Next, we determined the absorption coefficient of the exciton → molecule transition at ≈10⁶ cm^-1, an exceptionally high number attesting to the huge potential of exciton-related transitions in low screening 2D materials for opto-electronics. Moreover, assuming stimulated emission through the molecule → exciton transition, we obtain for the first time a parameter-free description of the material gain spectrum of CdSe CQWs that matches the experimental gain spectrum and accounts for the high gain coefficients and the remarkable lack of gain saturation. The theoretical model is disruptively different from gain in 0D QDs and adds new insight into efficient optical gain development in 2D materials in general.
Finally, we extend the study to (colloidal) perovskite quantum wells which show strong exciton-phonon coupling leading to a notion of 2D polarons, and show that the latter has a dramatic impact on the gain model and metrics, in particular leading to lower gain thresholds but smaller gain coefficients.

Spatial Atomic Layer Deposition (SALD) is a recent variant of ALD that offers fast processing, even at atmospheric pressure, while preserving the unique assets of ALD, namely, precise thickness control down to the nanometer, high-quality films even at low temperatures, and unique conformality. As a result, SALD is ideal for applications requiring high throughput at low cost, such as new generation photovoltaics, LEDs or packaging. In particular, the SALD approach based on close-proximity deposition heads is highly versatile since it can be easily customized by proper design of the deposition heads and because the deposition takes place in the open air without the need of any deposition chamber.
In my talk, I will briefly introduce the SALD technique and present the developments we are currently carrying out at LMGP. I will then present several examples of how SALD can be used to enhance the performance of different (opto)electronic devices thanks to the deposition of nanometric films.

Low-dimensional semiconductor nanostructures are critical components in many advanced technologies, spanning diverse scientific areas including electronic devices, photonics, and electrochemical applications. In many cases, novel synthetic methods that enable on-demand growth, controlled assembly, and precise placement of low-dimensional semiconductors are desired. Here, we discuss our recent work published in ACS Nano describing the on-demand, laser-driven growth of colloidal semiconductor nanowires in solution. This first-of-its-kind demonstration of photothermally driven, solution-based semiconductor nanowire growth, utilizing the controlled laser irradiation of colloidal nanocrystals, enables the on-demand, solution-liquid-solid growth of a variety of semiconductor nanowires in a flexible, widely accessible platform that may potentially be advantageous in a number of different technological areas. In addition, we discuss our related work recently published in Nature Communications, describing the light-driven, precision alignment and controlled end-to-end assembly of low-dimensional semiconductor nanowires in organic solvent systems. Together, these works demonstrate a pair of new strategies available for the on-demand, light-driven growth and precision placement and assembly of semiconductor nanowires, and we hope that that these examples will lead to wider utilization of light-based techniques for the growth and assembly of low-dimensional semiconductor nanostructures for a variety of emerging technological applications.

To solve modern challenges in sustainable energy generation and advanced sensing/lighting technologies, significant attention is directed toward semiconducting materials with distinct photonic and optoelectronic properties. Receiving particular attention are colloidal Quantum Dots (QDs), solution-processible, fluorescent nanocrystal analogs of II-VI and III-V semiconductors. Quantum confinement effects in QDs give rise to narrow and size-tunable emission linewidths, and desirable properties may be engineered with a host of core, shell, and surface-bound ligand compositions.^[1] Another compelling class of materials, Metal Halide Perovskites (MHPs), are a strong candidate for next generation solar cells. MHPs exhibit high optical absorption and low exciton recombination and also may be synthesized with cost-efficient solution techniques.^[2]

Modern optoelectronic devices benefit from the integration of specialized constituent materials to realize new functionality. Combining the tunability and precise emission of QDs with the intrinsic charge transport properties of MHP is a compelling avenue for advancing LED and solar cell technologies. Heteroepitaxy-enabled combination of QD and MHP pairs with well-matched lattices, results in a heterostructure now known as a Quantum-Dot-in-Perovskite (QDiP) solids.^[3] Initial investigations since 2015 have revealed promising characteristics and performance of heterostructures comprised of lead chalcogenide quantum dots (PbS/PbSe) integrated within methylammonium-lead-iodide (MAPbI₃) perovskite among other QD-perovskite pairings.^[3,4]

In this talk, we present a detailed analysis of local order in PbS-MAPbI₃ QDiP heterostructures via high energy synchrotron X-ray scattering. Three-dimensional diffuse scattering maps of single QDiP crystals reveal static and dynamic deviations from an ideal lattice-matched structrure by the way of local, nearly static lattice defects associated with the interface between QDs and the MHP. Comparison to diffuse scattering of the pure MHPs^[5,6] shows that the presence of the QDs quench the dynamic, local 2D sheets of tetrahedral MHP within a globally cubic phase. Comparison to simulations isolates the contribution of each individual molecular constituent to the local order.

[1] C. R. Kagan, E. Lifshitz, E. H. Sargent, and D. V. Talapin, “Building devices from colloidal quantum dots,” Science, vol. 353, no. 6302, Aug. 2016, doi: 10.1126/science.aac5523.
[2] X. Qi et al., “Photonics and Optoelectronics of 2D Metal-Halide Perovskites,” Small, vol. 14, no. 31, p. 1800682, Aug. 2018, doi: 10.1002/smll.201800682.
[3] Z. Ning et al., “Quantum-dot-in-perovskite solids,” Nature, vol. 523, no. 7560, pp. 324–328, Jul. 2015, doi: 10.1038/nature14563.
[4] H. Chen, J. M. Pina, Y. Hou, and E. H. Sargent, “Synthesis, Applications, and Prospects of Quantum-Dot-in-Perovskite Solids,” Adv. Energy Mater., p. 2100774, May 2021, doi: 10.1002/aenm.202100774.
[5] N. Weadock, private communication, Oct. 2021
[6] T. Lanigan-Atkins et al., “Two-dimensional overdamped fluctuations of the soft perovskite lattice in CsPbBr3,” Nature Materials, vol. 20, no. 7, pp. 977–983, Jul. 2021, doi: 10.1038/s41563-021-00947-y.

Nature Materials is a monthly multi-disciplinary journal aimed at cutting-edge research across wide-ranging spectrum of materials science and engineering, covering the aspects of synthesis/processing, structure/composition, properties, and application of the materials. The research areas we are covering spans from engineering and structural materials, organic and soft materials, biomaterials, and photonic, magnetic, and optoelectronics materials, just to mention a few.
In the realm of applied physics and devices, the topic that I am handling revolves around photovoltaic, neuromorphic computing, flexible electronics, etc. Moving forward, I am especially interested in attracting more scientific works on material discoveries and their clever unique application to meet the increasing demands for sustainable development, as documented in the 17 Goals of United Nations agenda, such as Clean Water and Sanitation, Affordable and Clean Energy, Sustainable Cities and Communities, Good Health and Well-Being, Climate Action, as well as Innovation and Infrastructure.
Subsequently I am going to elaborate more on: 1) what is sustainable development? 2) why is it important and what are the goals? 3) how can we make it happen and what are the challenges? We are aiming to curate contents dedicated to this area, and lastly a few published papers that meet these criteria will be presented to exemplify what we are looking for, such as energy harvesting and storage, catalytic and separation materials, biomedical products, low-powered devices, and smart building.

Halide perovskite semiconductors are making waves for their exceptional performance as photovoltaic materials, but since they are such profound optoelectronic materials, discovery is underway to elucidate additional breakthrough applications. In this talk, we will show how forming heterojunctions containing perovskite nanocrystals enable unconventional control over spin, charge and light.
In traditional optoelectronic approaches, control over spin, charge, and light requires the use of both electrical and magnetic fields. In a spin-polarized light-emitting diode (spin-LED), charges are injected, and circularly polarized light is emitted from spin-polarized carrier pairs. Typically, the injection of carriers occurs with the application of an electric field, whereas spin polarization can be achieved using an applied magnetic field or polarized ferromagnetic contacts. Here, we used chiral-induced spin selectivity (CISS) to produce spin-polarized carriers and demonstrate a spin-LED that operates at room temperature without magnetic fields or ferromagnetic contacts. The CISS layer consists of oriented, self-assembled small chiral molecules within a layered organic-inorganic metal-halide hybrid semiconductor framework. The spin-LED achieves ±2.6% circularly polarized electroluminescence at room temperature.[1]
Long-lived photon-stimulated conductance changes in solid-state materials can enable optical memory and brain-inspired neuromorphic information processing. It remains challenging to realize optical switching with low-energy consumption, and new mechanisms and design principles giving rise to persistent photoconductivity (PPC) can help overcome an important technological hurdle. Here, we demonstrate versatile heterojunctions between metal-halide perovskite nanocrystals and semiconducting single-walled carbon nanotubes that enable room-temperature, long-lived (thousands of seconds), writable, and erasable PPC. Optical switching and basic neuromorphic functions can be stimulated at low operating voltages with femto- to pico-joule energies per spiking event, and detailed analysis demonstrates that PPC in this nanoscale interface arises from field-assisted control of ion migration within the nanocrystal array. Contactless optical measurements also suggest these systems as potential candidates for photonic synapses that are stimulated and read in the optical domain. The tunability of PPC shown here holds promise for neuromorphic computing and other technologies that use optical memory.[2]
[1] Kim et al., Science 371, 1129–1133 (2021) 10.1126/science.abf5291
[2] Hao et al., Sci Adv. 2021 Apr; 7(18): eabf1959. 10.1126/sciadv.abf1959

Ruddlesden-Popper perovskites (RPP), a two-dimensional organic-inorganic hybrid perovskites, have emerged as a material class for next generation opto-electronic applications, due to defect-tolerance, solution process, band-gap engineering, and higher chemical stability compared to the three-dimensional perovskite. In particular, RPPs have shown a wider bandgap range (2 to 3 eV) than that of typical 2D transition metal dichalcogenides by controlling the number (n) of inorganic PbX₆ octahedron slabs between ligand layers, which is suitable for light-emitting device applications. While 2D materials are typically tolerant to surface defects, domain boundaries of RPP have been reported to possess a high degree of deep trap states. Therefore, various synthesis techniques including antisolvent vapor assisted crystallization, inverse temperature crystallization, and cation intercalation into pre-existing crystal have been developed to fabricate a large scale and high-quality RPP single-crystals. Although fabricated crystals by conventional methods have single domain in lateral direction, the crystals often contain stacks with different n values which can be considered as impurity phases.
In this contribution, we develop a synthesis process for inch-scale, phase-pure RPP single crystals based on the inverse temperature crystallization technique. Nucleation and crystal growth mechanism are investigated to minimize the impurity phase formation. Various types of RPP single crystals including PEA₂PbBr₄, PEA₂MAPb₂Br₇, PEA₂PbI₄, PEA₂MAB₂I₇, and PEA₂MA₂Pb₃I₁₀ are successfully fabricated using the designed crystallization method with a lateral crystal size of inch scale. The opto-electronic properties of fabricated RPP crystals are characterized via time-resolved photoluminescence spectroscopy and conductive atomic force microscopy. Grazing incident wide angle X-ray scattering measurement is carried out to characterize the strain in RPP layers as well as crystallographic properties. Furthermore, phase purity of the crystal as well as the pico-second scale carrier dynamics are characterized by the transient absorption spectroscopy, demonstrating a potential towards emerging opto-electronic applications.

Halide perovskites have emerged as a potential candidate for light emitters of the next generation light emitting diodes (LEDs) due to their narrow electroluminescence (EL) spectral line width, i.e., high color purity. There has been a remarkable progress in the performance of perovskite LEDs (PeLEDs): external quantum efficiencies (EQEs) of green and red PeLEDs were ~ 0.1 % in 2014 and now EQEs over 20% have been reported. However, pure-blue emitters (emission wavelength ~ 460 - 470 nm), which is required to fulfill the Rec. 2020 specification in blue region, show modest device performances compared to red and green counterparts. Despite the easy tunability of the bandgap of the perovskites by anion mixing, halide segregation of mixed-anion three dimensional perovskites (ABX₃) greatly reduces spectral stability during device operation. To improve spectral stability of pure-blue PeLEDs, we selected pure bromide (hence, no halide segregation), Ruddlesden-popper perovskite (RPP) as a light emitter. The general formula of RPPs are A’₂A_n-1B_nX_3n+1 and the optical properties including bandgap of the RPPs are tunable by the n value, the number of [PbX₆] octahedron slabs between the spacers consisting of A’ cations. Free-standing PEA₂MAPb₂Br₇ (n = 2) single crystals were synthesized by inverse temperature crystallization method. A thin layer of RPP perovskite was prepared via micromechanical exfoliation off PEA₂MAPb₂Br₇ single crystal and transferred onto a substrate coated with a hydrophilic charge transporting layer. The final PeLEDs yielded an EL emission at 440 nm with an extremely narrow EL width of ~14 nm. Details on thickness-dependent optoelectronic properties of RPP flakes and their effects on device performances will be discussed.

The precise stacking of different layered structures such as graphene and MoS₂ has reinvigorated the field of 2D materials, revealing exotic phenomena at their interfaces. These unique interfaces are typically constructed one monolayer at a time. By contrast, self-assembly is a scalable technique, where materials selectively form in solution. We recently reported a synthetic strategy for the self-assembly of layered perovskite–non-perovskite heterostructures into large single crystals in aqueous solution, using organic groups as structure-directing agents. I will present various layered heterostructures that form as an interleaving of perovskite slabs with a different inorganic lattice, previously unknown to crystallize with perovskites. To our knowledge, these compounds are the first layered perovskite heterostructures to form as single-crystals using organic templates. I will discuss how this interleaving of two different inorganic structures can markedly transform the band structure and optical properties. Given the technological promise of halide perovskites, our intuitive synthetic route sets the foundation for the directed synthesis of complex semiconductors in water.

Lead-halide perovskite nanocrystals (PNCs) are of great interest for use in photodetectors and light-emitting diodes (LED) because of their unique optical properties including high photoluminescence quantum yields (PLQYs), narrow photoluminescence spectra and shallow trap levels.[1,2,3] Despite these advantages, the perovskite lattice is very vulnerable to moisture, temperature, and/or UV light. Oleic acid (OA) and oleylamine, which undergo weak acid-base reactions, cause ligand desorption on the PNCs surface.[4] This reduces colloidal stabilities and optical properties of PNCs. To solve these problems, various surface defect passivation strategies have been proposed. Zwitterionic small molecules having intrinsic dipoles can be a promising ligand to achieve colloidal and chemical stabilities by tightly binding to the ion-rich surface of PNCs.[4] For this reason, zwitterionic ligands (ZLs) have been utilized in various applications, representatively LEDs.[5] Recently, the excellent photo-active properties of PNCs have also attracted great attention. Under light illumination, PNCs are used as a photocatalyst for polymerization, receiving light energy to form active radicals.[6] With PNCs as an initiator, direct photopatterning have been performed using an amine-based polymer containing a photo cross-linkable functional group as a ligand.[7] However, the use of polymeric ligand can be inefficient for the synthesis, purification, and patterning processes.
In this presentation, we introduce a multi-functional ZL simultaneously exhibiting strong electrostatic interactions and photo-curability under UV light, while improving colloidal stabilities of PNCs. By functionalizing the tail side of the ZL to be a UV-cleavage site, both the curing ability and strong electrostatic interactions could be achieved in the same ZL molecule. Due to the strong electrostatic interactions between ZL ligands, the gap between PNCs can be considerably reduced. In addition, the surface oleylamine was etched by the addition of OA to facilitate the binding between the ZL and the crystal surface.[8] As a result, the PLQY of the defect-passivated PNCs is improved by multiple times. Also, the ZL ligands were found to be present on the ligand shell as well as on the surface defects; this ZL ligand shell enhances self-assembly characteristics of the PNCs. Based on these effects, active radicals generated by UV light can be efficiently transferred from the PNCs to the ligand and from the ligand to the ligand through the ZL ligand bridge. Consequently, the resulting PNC films can be much denser, and the PNCs in the films can be more closely packed. Fine and bright patterns could also be obtained.[9]
[1] Dong, … Sargent et. al. Nat. Nanotechnol. 2020, 15, 668-674.
[2] Yang, … Jang et. al. ACS Appl. Mater. Interfaces, 2021, 13, 4374-4384
[3] Gong, … Wu et. al. ACS Nano. 2019, 13, 1772-1783
[4] De Roo, … Hens et. al. ACS Nano 2016, 10, 2, 2071–2081
[5] Krieg, … Kovalenko et. al. ACS Energy Lett. 2018, 3, 641-646
[6] Yuan, … Chen et. al. Angew. Chem. Int. Ed. 2020, 59, 22563-22569
[7] Ko, … Bang et. al. Nano Lett. 2021, 21, 2288-2295
[8] Pan, … Bark et. al. Adv. Mater. 2016, 28, 8718-8725
[9] Noh, … Jang et. al. in preparation.

All-inorganic metal halide perovskites, CsPbX₃, are considered as a suitable candidate for the next generation light emitting diodes (LEDs) due to its extremely narrow light emission spectrum. A more challenging, but technologically more significant, task is developing high-performance blue light emitters, in particular, pure-blue in the wavelength range of 460 – 470 nm. In CsPbBr3-based LEDs, the emission wavelength can be easily tuned to blue emission by partially replacing Br with Cl. However, CsPbBr_3-xCl_x has low external quantum efficiency (EQE) and poor emission spectral-stability due to phase separation caused by halide segregation during device operation. Here, we present a synthesis method to produce stable phosphonate-passivated single-halide CsPbBr₃ in the form of nanoplatelets that is capable of true-blue emission without the need for halide-substitution, i.e., free from the operational instability due to the halide segregation. Nanoplatelets with thickness of 3 monolayers were synthesized by emulsion ligand-assisted reprecipitation. These nanoplatelets yielded photoluminescence (PL) peak at 462 nm with an extremely narrow full width half maximum of 16 nm. To improve thermal stability of CsPbBr₃ nanoplatelets, we conducted ligand modification where long-chain alkyl carboxylic acidic ligands was replaced by phosphonate ligands in the synthesis step. The phosphonate-passivated emitters show a higher PL quantum yield (PLQY) of 80% and improved thermal stability--90% of the initial PLQY was maintained after annealing at 70 C for 5 minutes in ambient air, while a control sample without passivation PLQY degraded to 50% of the initial PLQY. With further engineering of the device structure such as the introduction of a conjugated polyelectrolyte layer between a hole transport layer and a CsPbBr₃ nanoplatelet emitter, a maximum EQE of 0.55% and luminance of 0.5 cd/m² at the emission wavelength of 462 nm was achieved. A possible mechanism behind the emission enhancement with the phosphonate-passivation and conjugated polyelectrolyte layer will be presented.

Metal halide perovskite nanocrystals (NCs) have attracted tremendous attention due to their unique optoelectronic properties. However, the problems of toxicity and instability of lead-based perovskite NCs limited their further use in various applications. Recently, lead-free heterometallic halide layered double perovskite NCs are explored to solve these problems by substituting the lead component and reducing the crystal structure dimensionality. The heterometallic halide layered double perovskite NCs with enriched compositional space and highly tunable properties are significant for both fundamental studies and technological applications. Here, we report the colloidal synthesis of a series of Cs₄Cd_1-xCu_xSb₂Cl₁₂ (0 ≤ x ≤ 1) heterometallic halide layered double perovskite NCs with controlled metal compositions. Both material characterizations and theoretical DFT calculations are investigated to provide deep insights on the composition-structure-property relationships of the layered double perovskite NCs, such as the transformation of crystal structures, electronic band structures, magnetic ordering, etc. Photogenerated charge carrier dynamics of the layered double perovskite NCs are studied using ultrafast spectroscopic techniques. We also demonstrate that such colloidal layered double perovskite NCs can be solution-processed into a high-speed photodetector, indicating their great potential in various optoelectronic applications.

Colloidal quantum dots (CQD) have recently attracted much attention for large-scale optoelectronic applications such as light-emitting diode (LED) and solar cell. This is mostly because there exist scalable solution-based synthetic routes for CQDs, of which size and band gap are readily controlled. Chalcogenide CQDs have been extensively studied for LED (CdSe, ZnSe) and solar cell (PbSe, PbS) applications, and environmentally-benign III-V (InP, InAs) CQDs have emerged in commercial applications. However, the ligand-surface interaction in the organic-inorganic hybrid systems has not been seriously discussed although it is very important for further development and deployment of the CQD technologies. CQDs generally consist of inorganic nanocrystal cores and surface-passivating organic ligands. Depending on the nanocrystal cores and their surface passivation, CQDs could have various shapes from cubic to tetrahedral, cuboctahedral, spherical, and even tetrapodal shapes. The stability and performance of CQD materials and devices could thus be sensitively subjective to the surface-ligand interaction depending on type, shape, and size of CQDs.

In this talk, I will discuss our recent understanding on how to stabilize CQDs by controlling surface chemistry, or surface-ligand interactions of IV-VI, II-VI, III-V, and halide perovskite CQDs based on first-principles density-functional theory calculations [1-6] with close collaboration with experiment. Based on these atomic understandings, we could provide new possibility to control CQD materials for more functionality and better stability in optoelectronic applications.

References
H. Choi, J.-H. Ko, Y.-H. Kim, and S. Jeong, J. Am. Chem. Soc. 135, 5278 (2013).
J. Woo, J.-H. Ko, J. Song, K. Kim, H. Choi, Y.-H. Kim, D. C. Lee, and S. Jeong, J. Am. Chem. Soc. 136, 8883 (2014).
K. Kim, D. Yoo, H.Choi, S. Tamang, J.-H. Ko, S. Kim, Y.-H. Kim, S. Jeong, Angew. Chem. 128, 3778 (2016).
J-.H. Ko, D. Yoo, and Y.-H. Kim, Chem. Commun. 53, 388 (2017).
D. Yoo, J. Y. Woo, Y. Kim, S. W. Kim, S.-H. Wei, S. Jeong, and Y.-H. Kim, J. Phys. Chem. Lett. 11, 652 (2020).
Y. Kim, H. Choi, Y. Lee, W.-K. Koh, E. Cho, T. Kim, H. Kim, Y.-H. Kim, H. Y. Jeong, and S. Jeong, Nature Commun. 12, 4454, (2021).

Cd-based nanocrystals, including zero dimensional quantum dots and two dimensional nanoplates, are promising solid materials that offer tunable optoelectronic properties. Their electronic structure and lattice dynamics are sensitive to the size, core-shell structure and ligands.
Using first-principles calculations, we show that CdS, CdSe, and CdTe nanoplates exhibit composition- and thickness-dependent phonon properties and exciton binding energies. More specifically, their phonon band center, phonon "band gap", dielectric screening, and exciton binding energy E_b can be tuned as function of the composition and layer thickness. As the anion goes from S to Te, the phonon band centers and band gap decrease due to softening of the lattice, and their exciton E_b decreases due to enhancement of the dielectric screening. When the thickness reduces for the same composition, the phonon band gap increases as a result of higher concentration of surface-localized phonon, while exciton E_b increases due to both reduced dielectric screening and enhanced overlap between the electron and hole wavefunctions. By linking the phonon energies and the exicton energies, the resonant exciton-phonon coupling can be tuned via both the composition and thickness, which is confirmed by Bethe-Salpeter-Equation calculations. These results have important implications for engineering thermal conductivity and carrier mobility in Cd-based nanocrystals.

Bi₂O₂Se is an emerging semiconductor material with proposed applications in a wide variety of fields ranging from thermoelectrics to ferroelectrics all the way to photoacoustic cancer imaging. Oxychalcogenide Bi₂O₂Se possesses a layered tetragonal structure (I4/mmm) consisting of alternating bismuth oxide (Bi₂O₂) and weakly bonded square arrays of Se layers making ultrathin structures nearing the monolayer limit possible. This makes Bi₂O₂Se a desirable alternative to typical inorganic 2D materials such as h-BN or graphene which do not simultaneously possess desirable properties such as stability, high mobility, and a suitable bandgap for electronic applications. Bi₂O₂Se has been shown to possess these requirements such as ultrahigh undoped electron mobilities > 20,000 cm² V^-1 s^-1 at low temperatures, a band gap of ~0.8eV and environmental stability. In this work we probe the origins of the ultrahigh mobility, doping response and future directions of Bi₂O₂Se electronics using a combination of self-consistent relativistic quasi-particle GW, hybrid functional density functional theory (DFT) calculations and ab-initio transport and scattering (AMSET) postprocessing.

Due to its superior optoelectronic properties, colloidal quantum dots (QD) have been receiving enormous attention from the research community as a strong candidate for a next-generation electroluminescent devices. Quantum mechanical spatial confinement allows tuning the optical bandgap of the material relatively easily, making them front runners for full-color flat panel displays, i.e. light emitting device (LED). At the present, the external quantum efficiency (EQE) of QDLED is on par with that of state-of-the-art organic LED. Although the EQE progressively approaches to the requirement for commercialization, device lifetime is still below what is required for commercial application. QD emission mechanism has mainly been focused to understand degradation mechanisms because QD is vulnerable to carrier-loss via Auger recombination process from unbalanced charge in QD. Recently, charge transport layers have been found to also play a major role in device degradation. More specifically, the organic material-based hole transport layer has been found to degrade by excitons or charges during electrical stress. On the other hand, the influence of electron transport layer (ETL), typically made of ZnO and thus assumed to play a little role in device degradation, has been overlooked. Recently, some reports suggested that ZnO ETL could deteriorate charge imbalance in adjacent QD layer due to its relatively higher carrier mobility than organic material-based hole transport layer. In addition, intrinsic defect states in ZnO can act as charge trapping sites. In this regard, some reports suggested that modification of ZnO strongly affect device performance, both efficiency and stability. This modification usually includes the use of a bilayer form of ZnO or a functional layer or metallic anion doping, e.g. aluminum, magnesium, in ZnO layer. These approaches somehow effectively control device efficiency but there are still many challenges in terms of stability in device operation. In this report, we will discuss different unique strategies for ZnO modification with a focus on improving device stability. Various doping techniques in ZnO effectively modify electrical and optical properties of ZnO and improve both device efficiency and stability significantly. Especially, device stability improves more than 10 times than a control device. Various electrical, optoelectrical, structural and spectroscopic observations in the experiments will be discussed to shed light on new strategies for improving device stability and performance.

Colloidal quantum dots (QDs) are semiconductor nano particles that have unique electrical and optical properties. Due to the quantum confinement effect, QDs have huge benefit on color-tunability through particle size control and excellent color gamut. These advantages make quantum dot light-emitting diodes (QLEDs), using QDs as the emission layer (EML), to get significant attention as a next-generation display technology. However, most previous QDs studies have involved cadmium chalcogenide based materials. Due to RoHS regulation, electrical and electronic equipment is restricted from the use of Cd. Indium phosphide (InP) QDs are a promising candidate to replace Cd based QDs as an enviromrentally non-toxic meterials. Currently, InP QDs using Tris(trimethylsilyl)phosphine (P(TMS)₃) as the phosphorus (P) precursor have a high photoluminescence quantum yield (PL QY) and a narrow full width at half maximum (FWHM).[1] However, the use of P(TMS)₃ in the synthesis of InP QDs is very cautious because it is explosive, toxic, and even expensive. In addition, a rapid nucleation rate results in the ununiform size distribution. Various researches have been conducted to replace P(TMS)₃ by P precursors with high stability and moderate reactivity. Tris(diethylamino)phosphine (P(DEA)₃) is the one of the strong candidates satisfying the above conditions.
In this work, green InP/ZnSeS/ZnS QDs were synthesized successfully by using P(DEA)₃, zinc chloride (ZnCl₂), and indium(III) iodide (InI₃) as precursors. The InP/ZnSeS/ZnS heterostructure was adopted to decrease the lattice mismatch (7.85%) between InP core and ZnS outer shell.[2] The 1-octanethiol ligand was used to form short and reactive ligands on the ZnS shell rapidly.
The specific conditions including temperature and reaction time for core synthesis, ratio of P and I precursors were optimized to improve the PL QY of InP QDs. The resulting QDs showed emission wavelength at 525 nm with FWHM of 44 nm and PL QY of 80 %. InP QDs were applied as an EML in the QLEDs and their electroluminescent characteristics were investigated systematically. The device consisted of ITO/PEDOT:PSS/TFB/InP QDs/ZnO/Al and exhibited a peak luminance and current efficiency of 450 cd/m² and 5.4 cd/A, respectively. Additional gallium dopping was adopted to improve the PL QY of InP QDs. Furthermore, we plan to optimize the device fabrication by applying different inorganic ETL.
[1] Li, L.; Reiss, P. One-pot Synthesis of Highly Luminescent InP/ ZnS Nanocrystals without Precursor Injection. J. Am. Chem. Soc. 2008, 130, 11588−11589
[2] S. Kim, T. Kim, M. Kang, S. K. Kwak, T. W. Yoo, L. S. Park, I. Yang, S. Hwang, J. E. Lee, S. K. Kim, and S. W. Kim, Highly luminescent InP/GaP/ZnS nanocrystals and their application to white light-emitting diodes., J . Am. Chem. Soc. 134, 2012, 3804.

Colloidal quantum dots (QDs) have been attracting attention for next generation display due to unique electrical and optical properties through quantum confinement effect. Recently, quantum dot light-emitting diodes (QLEDs) using their electroluminescence show remarkable performance, but the use of Cd-based QDs are limited by restriction of hazardous substances (RoHS) regulation. For this reason, recent research is focused on replacing Cd-based QDs by Indium phosphide (InP) based QDs.
Common InP QDs have ZnS as an outer shell to passivate the surface of core. Additionally, other studies reported ZnSeS as an inner shell to minimize the lattice mismatch between core and outer shell.¹⁾
Conventional InP QDs synthesis uses tris(trimethylsilyl)phosphine (P(TMS)₃) for phosphorous precursor which has issues of uncontrollably high reactivity and toxicity, resulting in the broad size distribution.²⁾ Therefore, other aminophosphines such as tris(dimethylamino)phosphine (P(DMA)₃) and tris(diethylamino)phosphine (P(DEA)₃) have been studied as alternatives to P(TMS)₃.
In this study, we synthesized the blue multishell InGaP/ZnSeS/ZnS QDs by a hot-injection method using P(DMA)₃ as a precursor of phosphide. InGaP cores were synthesized through a cation exchange method, then passivated with ZnSeS as an inner shell and ZnS as an outer shell. We also adopted a two-step approach to synthesize QDs with high color purity by reducing the size distribution of the core. The QDs synthesized through the size sorting process showed a narrower FWHM than the unsorted QDs. Meanwhile, various ligand materials of the outer shell were applied to produce a higher photoluminescence quantum yield (PLQY). Conventional ligand material of ZnS which is the outer shell, oleic acid (OA) was exchanged with other carboxylic acids such as palmitic acid and hexanoic acid. The synthesized QDs showed a correlation between PLQY and the hydrocarbon chain length of the ligand, with over 70% of PLQY and about 40 nm of FWHM. The inverted structured QLEDs using blue InGaP-based QDs were fabricated, and their electrical characteristics were analyzed. The best devices showed a high luminance of 350 cd/m² and it is the promising candidate for non-toxic blue QDs.
[1] Wegner, K. D.; Pouget, S,’ Ling, W.L.; Carrière, M.’ Reiss, P. Gallium – a versatile element for tuning the photoluminescence properties of InP quantum dots. Chem. Commun. 2019, 55, 1663-1666
[2] Kim, Y.; Yang, K.; Lee, S. Highly luminescent blue-emitting In1-xGaxP@ZnS quantum dots and their applications in QLEDs with inverted structure. J. Mater. Chem. C, 2020, 8, 7679-7687

Indium arsenide (InAs), one of the III-V compound semiconductors, has a relatively small carrier effective mass, direct bandgap, and small exciton binding energy, which are attractive properties for state-of-the-art electronic and optoelectronic applications. Due to the quantum confinement effect, the nano-sized crystals of InAs can possess additional benefits, including tunable optical bandgap ranging from near-infrared (NIR) to short-wave infrared (SWIR) and versatile solution processability. Given these advantages, the IR-responsive InAs quantum dots (QDs) can be integrated as a thin solid form into various IR photosensors, enabling to apply machine vision, imaging sensors, and telecommunications. For high performance of QD-based photodetectors, it is important to obtain highly monodisperse InAs QDs and control the surface defects that can enhance the charge transport properties in QD films.
The synthesis of InAs QDs is not, however, as simple as the ionic compound semiconductor nanocrystals, because of their strong covalent bonding nature and the lack of group 15 precursors. Previous research using continuous injection and seeded growth methods have been focused on reactivity control of highly reactive silylated or germylated arsenic precursors, which have difficulty handling to some extent due to toxicity and pyrophoric properties of the precursors. Meanwhile, co-reduction methods with commercially available arsenic precursors are another facile approach for InAs QD synthesis. The simple heat-up process along with reducing zinc and arsenic precursors together simultaneously, may be suited for large-scale production, but it normally requires a subsequent size selection process because of a broad size distribution. Additionally, as the nanoscale III-V semiconductors suffer from the oxidative defects, the surface of InAs QDs is also prone to oxidation. It is required to eliminate the oxidative defects so that the QDs are electronically connected for efficient charge extraction without charge trapping.
In this work, we propose the synthesis approach to improve monodispersity of InAs QDs based on the co-reduction method. We introduce zinc halide precursors as a form of coordination complex into the indium and arsenic reaction mixtures. During the synthesis, the zinc ions are chemically passivating the surface of InAs QDs, allowing to induce size-focusing as well as to remove surface defects. The surface oxide-free InAs QDs are stable enough as a colloidal form even with weakly bound ligands to facilitate the follow-up process. When InAs QDs are integrated into the infrared photodiode as an IR absorber, the surface zinc ions can electronically modulate the energy levels and carrier concentrations. The IR photodiodes with InAs:Zn QD layers exhibit higher IR sensitivity and faster response than the bare InAs QDs. We anticipate that the unique synthesis approach for InAs QDs with electronically active zinc ions should inspire QD research into a new direction for NIR and SWIR applications.

Colloidal Quantum Dots (CQDs) which exhibit the bandgap transition in the infrared region have been of great interest due to various applications. In addition, the need for nontoxic nanomaterials in the infrared range has been required. Here, we present the Ag_xTe (x>2) CQDs which have the excitonic transition from 1.8 to 2.7 μm by varying the ratio of Ag/Te, resulting in the great air stability and strong emission intensity. The enhanced emission intensity and improved structural rigidity by the Ag₂S shell growth over the Ag_xTe core are carefully investigated by X-ray photoelectron spectroscopy (XPS) of sulfur 2p. Furthermore, the photocurrent and current density-voltage curve of Ag_xTe CQD devices are successfully measured with an infrared light source at various temperatures. The responsivity of the detector at 0.63 V is 2.1 A/W at 78 K. The nontoxic Ag_xTe QDs in the infrared range which show stable optical properties and photocurrent results will open a great opportunity for solution-based infrared applications such as telecommunications, bioimaging, and infrared optoelectronics.

Quantum dot (QD) light-emitting diodes (QLEDs) have attracted attention as one of the promising candidates for next-generation displays. In QLEDs, device efficiency tends to be limited by inefficient hole injection induced by large energy barrier between a QD emission layer (EML) and a common hole transport layer (HTL). The energy barrier can be tuned by adopting p-type semiconductor materials with proper valence band maximum (VBM) energy levels close to the QD EML. Herein, we introduce efficient InP-based red QLEDs in an inverted structure with p-type inorganic nanoparticles within the QD EML. Ultraviolet photoemission spectroscopy (USP) measurement revealed that the VBM of the QD EML was upshifted by 0.2 eV from 6.7eV to 6.5eV when nickel oxide nanoparticles were mixed into the QD EML, which leads to improvement of the hole injection and suppression of the excess electron injection into QDs. As a consequence, we successfully demonstrated QLEDs exhibiting a maximum external quantum efficiency of 4.7% and a maximum luminance of 16770 cd/m2, which are higher by 1.5-fold and 1.4-fold, respectively, compared with those of pristine QLEDs. The results elucidate that introducing a small amount of p-type inorganic nanoparticles into the QD EML could be a simple and effective way to improve hole injection in QLEDs.

Quantum dot displays (QLEDs) are emerging as next-generation displays due to their narrow emission linewidth, tunable emission wavelength, high cost effectiveness, high color purity, fast response speed and high photostability. Most quantum dot displays use organic charge transport layer materials, which are expensive, weak to moisture and oxygen, and weak to high current density. Nickel oxide (NiO_x) is a next-generation hole transport layer for quantum dot displays because it has a low price, high stability, high light transmittance. However, most nickel oxide thin films require high temperature post heat treatment to form a NiO_x thin film by heat-treating the precursor. There is a disadvantage in that it is difficult to make a thin film of uniform film quality due to an uneven particle distribution because there is no surfactant used for synthesis and there is no good dispersion due to agglomeration problem caused by calcination. To solve these problems, Oleylamine, Oleic Acid, and Borane Trimethylamine were used as surfactants and reducing agents in solution process by controlling the temperature and the amount of Borane Trimethylamine in an air atmosphere was controlled by synthesis of NiO_x nanoparticles. After dispersing the synthesized NiO_x and thinning it through spin coating to analyze the physical properties, it was applied as a hole transport layer of a quantum dot display to evaluate its properties. It was confirmed through XRD and TEM that NiO_x nanoparticles were dispersed with a particle size of about 4 nm. A thin film was deposited by spin-coating a NiO_x solution, and transmittance, absorbance, XPS, and UPS were analyzed according to post-heat treatment conditions and UV Ozone Treatment. As the heat treatment temperature increases, the transmittance tends to increase and the transmittance tends to decrease after UV ozone treatment. This is because the amount of Nickel Oxide Hydroxide(NiOOH) increased according to the change of Ni²⁺ → Ni³⁺. As a result of analyzing the UPS data, it was confirmed that UV Ozone treatment was necessary for the hole transport layer performance of the quantum dot display as it became a deeper Work function and Valance Band Maximum after UV Ozone Treatment. Afterwards, a quantum dot display was fabricated and the performance such as Current Efficiency, luminance, and internal quantum efficiency according to the change of conditions of QLED.

NIR photodetection is the key technology for emerging applications such as bioimaging, autonomous driving, and augmented reality. Owing to their tunable bandgap, solution processibility, and non-toxic components, InAs colloidal quantum dots (CQDs) are attractive materials for the NIR photodetector. Recently, there have been attempts to employ InAs CQDs solids as active layers in p-n junction photodiodes. [1] For these devices to achieve high performance, control over carrier type, concentration, and removal of traps are fundamental but it remains a challenge since InAs CQDs are generally reported to exhibit n-type with high trap density. [2] Here, we incorporated environmentally benign Zn on InAs CQDs via post-synthesis doping process. Through this treatment, we were able to switch the n-type property of original InAs CQDs into p-type with a controlled doping level, demonstrated by field-effect transistors, and inductively coupled plasma mass spectroscopy. Then, using the Zn doped InAs CQD solids capped with short ligands, we fabricated InAs CQD photodiode including n-type metal oxide for the electron transport layer. To study the impact of doping, the device with n-type InAs CQD was also implemented. Finally, device performance was compared evaluating the current-voltage curve, external quantum efficiency, and detectivity. We believe that this study introduced an effective strategy for improving the performance of nanocrystal-based optoelectronic devices and laid the groundwork for their commercialization.
[1] Nano Lett. 2021, 21, 14, 6057–6063
[2] Nat comm 9, 4267 (2018)

Quantum dots light-emitting diodes (QD-LEDs) have been considered as a next-generation display device due to their excellent optoelectronic properties such as high color purity covering wide color gamut. To improve the luminescent efficiency of QD-LEDs, many efforts have been focused on the formation of heterojunctions in QDs. The well-defined heterostructures can offer unique opportunities to impart new and/or improved optoelectronic properties. To understand the behaviors of charge carriers, we need to analyze the charge carrier dynamics qualitatively and quantitatively. Herein, we fabricate QD-LEDs with two different quasi-type-II QDs by growing different shells on the same core. The first QDs can efficiently distribute electrons and hold the holes in the narrow bandgap core due to the large valence band offset at the core/shell interface, while the second QD can do the opposite. Each QD acts as a donor and an acceptor, manipulating electrons and holes, respectively. These quasi-type-II QDs can exert different electronic actions in optoelectronic devices where the complicated interplay between both injected charge carriers occurs. When these QDs are incorporated into the LED devices, each device shows different charge injection and charge transport due to the energy offset between charge transport layers and an emitting layer. We can investigate charge imbalance in various operating conditions allowing us to improve the device stability and performance.

All-inorganic CsPbI₃ perovskite quantum dots (PQDs) are attracting extensive attentions owing to excellent optical and electrical properties. Especially, CsPbI₃ PQDs have many advantages as a solar cell absorber because of tunable bandgaps, high solar light absorbance, and the feasibility of layer-by-layer film deposition with solution processes, enabling high power conversion efficiencies. Despite the advantages, the synthesis conditions of CsPbI₃ PQDs by typical hot-injection methods still require synthetic temperatures over 160 ^oC and vacuum processes, which hinder the scale-up synthesis of the PQDs.[1] Therefore, a facile and low energy synthesis method for CsPbI₃ PQDs needs to be developed for the mass production of CsPbI₃ PQD solar cells. To lower the energy consumption for the CsPbI₃ PQD synthesis, ligand-assisted reprecipitation (LARP) method can be a promising route because it enables the synthesis at low temperatures without vacuum processes, thereby reducing processing costs and time.[2]
In this presentation, a facile way is developed to enhance the stabilities and production yields of the cubic CsPbI₃ PQDs synthesized using a reported LARP method (named as ‘unconventional LARP’ method) at a low temperature without vacuum processes. [3] However, the CsPbI₃ PQDs synthesized by the ‘unconventional LARP’ method are easily decomposed during washing steps with any of polar non-solvents. To circumvent the issue, degradation mechanisms of the PQDs are systematically studied focusing on the surface of CsPbI₃ PQDs by using combinatory analyses of X-ray diffraction, nuclear magnetic resonance spectroscopy, and density functional theory calculation. As a result, CsPbI₃ PQDs synthesized by the LARP method could be successfully purified using a polar non-solvent without degradation, and the washed CsPbI₃ PQDs maintained their cubic phase crystallinities and photoluminescence emissions for a long time. Based on these findings, precursors for the PQD synthesis are controlled to enhance the production yields and stabilities, enabling liter-scale large amount syntheses. From this approach, high-concentration solutions of cubic CsPbI₃ PQDs could be prepared for the fabrication of solar cells exhibiting over 8 % of power conversion efficiencies.
[1] A. Swarnkar, … J. M. Luther Science 2016, 354, 92-95.
[2] H. S. Yang, … J. Jang ACS Appl. Mater. Interfaces 2021, 13, 4374-4384.
[3] H. Huang, … L. Polavarapu Angew. Chem. Int. Ed. 2019, 58, 16558 –16562

Nanocomposites comprised of anisotropic liquids and nanomaterials are appealing for realizing tunable optical properties. The unique assembly properties of anisotropic nanomaterials [1] makes them useful building blocks in creating novel optically active nanocomposites. However, literature reports on optically active nanocomposites comprised of liquid crystals (LCs) and 2D nanomaterials are rare.
We will present our work on the assembly of 2D CdSe semiconductor nanoplatelets in chiral LCs. Thermotropic cholesteric LCs were used to realize chiral nanocomposites having temperature-driven chiroptical properties. Optically active nanocomposites were achieved at the LC cholesteric phase while optically inactive composites were observed at the LC isotropic phase. The chiroptical properties of nanoplatelets comprised of different monolayers will be presented. In addition, thin films of these composites were successfully prepared using polymer stabilization and the characterization of these will be presented as well. This work demonstrates a unique and facile approach to achieve optically active nanoplatelet-LC composites with tunable chiroptical properties. From an application standpoint, such chiral materials are invaluable in polarization-sensitive optoelectronic applications.
References
1) J. Phys. Chem. Lett. 2016, 7, 632−641

Bismuth based semiconductors have emerged as promising candidates for applications in many fields, such as photocatalysis, photodetection, and solar cells, among others. BiSI in particular, is one of the most recently studied. With a band gap of 1.57 eV, comprising elements which are not in a critical supply state, and a preferential electronic conduction along its c-axis in the crystal structure, this semiconductor is gaining particular attention. Furthermore, its band structure, similar to lead perovskites, makes it defect tolerant, a most desirable property for electronic applications. However, given its ternary nature, the synthesis of this compound presents several challenges, the most important being the competition of other iodided compounds such as BiOI or BiI₃. This work presents the optimization of the synthesis of BiSI nanorods (NR) in a solution method, dispensing the equipment needed for a solvothermal synthesis, the most popular reported when dealing with BiSI nanostructures, and considerably reducing reaction times. Bi₂S₃ nanoflowers (NF) and powdered I₂ as precursors provide the elements Bi and S, and I, respectively, limiting the by-products generated during the reaction. As for the solvent, mono ethylene glycol (MEG) proved to be the best option for a pure yield of BiSI NR. This signifies not only the use of a green solvent, but the avoidance of complicated media such as H₂S currents. We also study the formation mechanism of BiSI nanorods, which takes place from the self-sacrificing template of Bi₂S₃ nanoflowers. This template acts in a synergistic way with the I₃^- species formed from the interaction of MEG and I₂. There is an interaction with the double chain of Bi₂S₃ to form the double chain of BiSI. This reaction takes place at mild temperatures (180^oC) and the product obtained is easily extracted by centrifugation and washing with ethanol. The speciation of bismuth in the reaction media was also studied, determining that a Bi-O-I complex is in solution, preventing the competition of BiOI solid phase. In order to elucidate this mechanism we made several experiments: varying the reaction time, temperature, precursor’s morphology and/or chemical speciation, among others. XRD with Rietveld refinement, SEM and TEM with EDS mapping, HR-TEM, UV-Vis and IR spectroscopy, and Confocal Raman microscopy aided in the characterization. The obtained nanorods were suspended in isopropanol and drop casted onto a glass substrate in order to get a homogeneous black film. This film was characterized by UV-Vis spectroscopy to analyze the optical characteristics and a band gap of 1.6 eV was observed, in accordance to reported values. This value makes BiSI nanorods a potential solar cell absorber material. In summary, we not only present an easy, Green, and scalable synthesis of a novel material, but also an insight into the chemistry of bismuth based semiconductors that could be applied to similar compounds. In addition, the optical properties of the BiSI nanorods show its potential in photovoltaic applications.

Semiconductor nanoplatelets (NPLs) are a class of nanocrystal with precise atomic thickness (1-2nm) and large lateral areas (hundreds of nm). Control over the number of monolayers (ML) in these materials offers exceptional control over their size-confined optical properties. Their unique morphology allows for interesting properties arising from their surfaces and lateral extent, but synthetic control over their lateral size has not been robustly explored in the literature. We demonstrate a controlled seeded growth for mesoscale CdTe NPLs (>1000nm), exploring parameters for optimization and in-situ monitoring to elucidate the mechanism of growth. Furthermore, using these mesoscale NPLs, we demonstrate correlated electron and optical microscopy to study single NPLs, drawing comparisons to 2D semiconductors.

The oxidation of graphene is an excellent method to control the electrical characteristics of graphene-based devices. In order to oxidize the CVD graphene surface, dry-oxidation using ozone/ultraviolet and oxygen plasma has been performed, however, it causes the increase of manufacturing cost for graphene-based electronic devices. Here, we report the reliable surface oxidation of the CVD graphene by wet-based oxidation using potassium permanganate (KMnO₄)/diluted sulfuric acid (H₂SO₄). The oxidized graphene is successfully synthesized without pores and peeling off using varied H₂SO₄ concentrations. It is clearly observed that the oxygen-related functional groups, such as epoxide/hydroxyl, carbonyl, and carboxyl groups are increasing as a function of the increased H₂SO₄ concentrations. The I–V curves of graphene oxidized by 30% H₂SO₄ only shows the nonlinear curve like a diode, which is attributed that the oxidized graphene is composed of nano-sized sp² (electrically conductive region) and sp³ (electrically insulated region) areas, which results in the carriers transport by the direct tunneling through the interfaces. Hence, our wet-based oxidation technique devotes the fabrication of graphene-based applications owing to the simple control of the electrical characteristics.

Ultra-clean van der Waals (vdW) contacts on two dimensional (2D) transiiton metal dichalcogenides (TMDs) are essential for enabling several different types of high performance electronic devices. However, it has been challenging to make p-type contacts on 2D TMDs. I will present our recent work on making clean p-type contacts on 2D TMD using high work function metals such as Pt and Pd. The realization of p-type devices allows their integration with vdW contacted n-type devices. I will briefly describe our attempts to realize vertically integrated p-n junction devices. In addition, we have realised ferromagnetic (FM) vdW contacts. We have attempted to use the vdW gap between 2D materials and FM contact as a tunnel barrier for spin injection. Finally,I will discuss the results of vdW contacts on ferroelectric 2D materias to realize tunnel junciton devices.

Two dimensional (2D) electron transport has been one of the most important topics in science and technology for decades. It was originally studied in 3D semiconductors and then continued in 2D van der Waals (vdW) crystals. In this talk, I will start with the large-scale processes for generating 2D crystalline semiconductor transition metal dichalcogenide (TMD) films and superlattices that could be used to fabricate atomically thin integrated circuits cleanly. These large scale TMD films also display novel optical properties depending on the geometry of the film itself and on the geometry of the incident light. I will discuss two examples. First, TMD films grown with nanoscale textures produce optically isotropic films with strongly enhanced absorption. Second, TMD monolayers can be used to form three-atom-thick waveguides with a long (~ 1 cm) propagation length that can be used to generate a novel 2D photonics platform.

A femtosecond-laser technique is presented for the fabrication of 2D nanoparticles. Flakes of 2D materials are dispersed in different solvents and irradiated with a femtosecond laser for different durations at different powers. The laser irradiation results in breaking of the flakes into smaller nanoparticles. Simultaneously, laser-dissociated solvent atoms can bond with the nanoparticles to form functionalized 2D nanoparticles, where the choice of solvent determines the nature of the functionalization [1]. Functionalization of different 2D nanoparticles, including graphene, graphene oxide, molybdenum disulfide, tungsten disulfide, and boron nitride, with carbon, oxygen, nitrogen, and halogens is demonstrated. Under some conditions, conversion of the starting 2D material is possible. For example, the transformation of MoS₂ powder to sub-stoichiometric MoO_3-x nanosheets is demonstrated [2]. The functionalized 2D nanoparticles can be tailored for various applications including gas sensing [3], photothermal cancer therapy [2], fluorescence chemical sensing, and photovoltaics [4]. The incorporation of halogenated graphene particles into perovskite solar cells will be discussed in detail. The graphene particles are found to improve the humidity resistance of the perovskite solar cells and simultaneously enhance hole extraction.

[1] K. Ibrahim et al. Small 15:1904415 (2019).
[2] F. Ye et al. Chem. Mater. 33:4510 (2021).
[3] K. Mistry et al. Adv. Mat. Tech. 5:2000704 (2020).
[4] K. Ibrahim et al. Solar Energy 224:787 (2021).

Colloidal Quantum Dot (CQD) thin films have numerous applications in areas such as photovoltaics, photodetectors, and light emitting diodes. However, the hole transport layer (HTL) in the state-of-the art architectures for these devices has been identified as one of the limiting factors in performance. The current ethanedithiol-passivated PbS CQD-based HTL (PbS-EDT), has been shown to include a high density of electron trap states and degrade the underlaying absorbing layer. Because of these limitations, we sought to find an alternative material to replace or augment the existing PbS-EDT HTL. Two-dimensional transition metal dichalcogenides (TMDs) are atomically thin semiconductor materials that exhibit interesting and tunable electronic and optical properties with applications for optoelectronic devices. Due to these promising properties we sought to incorporate tungsten diselenide (WSe₂), a p-type TMD with high carrier mobilities and similar valence band edge energy to the current PbS-EDT HTL, into the current state-of-the art PbS CQD solar cell architecture to improve photovoltaic performance.

We use 1D optoelectronic simulations to investigate the expected performance of different combinations of PbS-EDT materials with 2D WSe₂ as well as completely replacing the PbS-EDT HTL with 2D WSe₂. Because the optical and electronic properties of WSe₂ are strongly dependent on the number of TMD monolayers, we also performed simulations testing solar cell performance as a function of WSe₂ thickness. We found that structures with both a combination of PbS-EDT with 2D WSe₂ as an enhancer and structures having solely several monolayers of WSe₂ as the HTL replacement displayed increased performance conversion efficiencies (PCEs) compared to control devices. This confirmed that 2D WSe₂ is a promising candidate to augment as well as a replace the PbS-EDT HTL.

Based on our simulation results, we sought to develop a method to incorporate WSe₂ into the existing PbS CQD solar cells architecture. Conventionally, TMD nanosheets are grown via chemical vapor deposition or molecular beam epitaxy, both of which require growth temperatures not compatible with the current PbS CQD solar cell materials. We instead used a solution-based liquid-exfoliation method to incorporate 2D WSe₂ into the solar cell architecture. Using liquid exfoliation allowed us to use low temperature spin-casting methods to deposit films of WSe₂ nanoflakes on our heat-sensitive CQD solar cell films. We fabricated structures with 2D WSe₂ on either side of the existing PbS-EDT layer. We used current density–voltage (J-V) measurements under AM1.5G illumination to characterize the performance of the solar cells, ultraviolet–visible (UV-vis) spectrophotometry to characterize the absorbance spectra of the WSe₂ nanoflake suspension, and transmission electron microscopy (TEM) and scanning electron microscopy (SEM) to analyze the morphology of our exfoliated and deposited nanoflake films. We saw an absolute increase in PCE of 1.3% using a structure with 2D WSe₂ on the back side of the PbS-EDT and an absolute increase of 0.8% PCE in the structure with WSe₂ on the front side of the PbS-EDT. The increase in performance is mainly attributed to an increase in fill factor and optimized band edge energy alignment in the device.

We showed that solution-processed 2D WSe₂ is a suitable material for enhancing the HTL of PbS CQD solar cells. We were able to achieve an efficiency increase from 8.4% to 9.7% PCE by incorporating 2D WSe₂ into the current state-of-the art PbS CQD solar cell architecture. Liquid-exfoliated WSe₂ nanoflakes are a promising material for enhancing the performance of PbS CQD solar cells and could be a viable material for improving other PbS CQD-based optoelectronics.

The unique physical properties of low-dimensional metal sulfide materials have attracted great attention to increase the efficiency of solar energy conversion into electrical energy via (photo)electrochemical reactions and photovoltaic. The ultrahigh surface area of nanostructured semiconductors in combination with their extraordinary physiochemical, electronic and optical properties offer the application as (photo)electrocatalysts.The huge number of active sides of 2D van der Waals materials like transition metal disulfides (TMDC) and monosulfides (MS), as well as their suitable and tunable band gap offer their application in energy conversion devices. The lacking control of large-scale material synthesis corresponding to specific requirements in commercial material formation processes is still challenging, which motivated us to develop a unique synthetic approach to low-dimensional layered materials MS₂ (M= Mo^IV, W^IV, Ti^IV, Nb^IV, Ta^IV, Sn^IV), MS (M=Sn^II, Ge^II). A uniform synthesis route of molecular building blocks for controlled formation of (air)stable precursor classes [M{S(C₂H₄)₂NMe}_x] (M = Mo^IV, W^IV, Ti^IV, Sn^IV, x = 2; M = Ge^II, Sn^II, x = 1) was developed. Following a simple synthetic protocol, the reaction of tridentate SNS donor ligand with suitable metal compounds resulted in (air)stable molecular precursors. Their simple thermal decomposition enabled the targeted formation of homogeneous crystalline 2D MoS₂, WS₂, TiS₂, NbS₂, TaS₂, SnS₂ and SnS. The wet chemical syntheses via microwave assisted decomposition of tin based precursors resulted in SnS and SnS₂ particles.
These molecular building blocks provide advances in the synthesis, characterization and applications of these low-dimensional materials for (opto)electronic applications.

The 3D printing of 2D materials offers a cost-efficient and sustainable route for the fabrication of integrated electronics and advanced energy devices with new architectures.
In particular, the assembly of 2D materials into rationally designed three-dimensional electrodes can enhance the charge transport kinetic and increase the surface area, resulting in superior (photo)electrochemical performance. In this talk, I will discuss 3D printed photo-chargeable microdevices obtained from aqueous inks of graphene and transition metal dichalcogenides (TMDs) respectively without the need of functional additives or high-temperature processing. We discuss TMDs synthesis from molecular precursors, inks formulation and device performance within the context of the state- of-the-art of photo-chargeable devices.

Quantum dot light-emitting diodes (QLEDs) has gained tremendous response in next-generation display and lightening applications because of their outstanding properties such as high color purity, wide color gamut, and simple fabrication process.^[1] The performance of environmentally friendly Cd-free (InP) QLED is lagging far behind compared to toxic Cd-based QLEDs. In many QLED devices, zinc oxide (ZnO) was used as an electron transport layer (ETL) due to high electron mobility, high transparency, and simple solution processability. The high electron mobility of ZnO ETL causes charge unbalance in the QLED device, which limits the device performance.^[2] To modulate the ZnO ETL mobility many reports have been published in the literature on Cd-based QDs.^[3] However, there is still room to improve charge balance in the QLED devices by limiting the electron transport.
In this study, we report high efficiency and long lifetime inverted red InP-based QLED devices by using slow mobility inorganic KHU-ETL as interlayer. To see the effect of interlayer on device performances, we have fabricated the device with the following device structure: ITO/ZnO/KHU-ETL as interlayer/red InP/ZnSe/ZnS QDs/DBTA/PCBBiF/HATCN/Al and also fabricated control device (without interlayer) for effective comparison. The optimized QLED device shows maximum current efficiency (CE) of 10.8 cd/A and maximum external quantum efficiency (EQE) of 10.6% with an EL peak at 630 nm and CIE color coordinates of (0.68, 0.31). This maximum CE and EQE values are almost 2.2-fold improved compared to the control device (i.e, CE: 4.8 cd/A and EQE: 4.5%). The optimized device also showed a half lifetime (LT₅₀) of 480 hours at an initial luminance of 1000 cd/m². By assuming acceleration factor n=1.8, the half lifetime at an initial luminance of 100 cd/m² is 30,285 hours. To know the exact reason behind the improvement of device efficiency and lifetime, fabricated electron-only devices (EOD) without and with KHU-ETL interlayer and compared with hole-only device (HOD). With the insertion of the interlayer, the electron current densities greatly reduced and improved the charge balance in the InP QLED. But still, there is a mismatch between EOD and HOD current densities. To further improve the charge balance, we adopted QD:KHU-HTL doped emissive layer (EML) approach for efficient hole injection. The deep highest occupied molecular orbital (HOMO) of KHU-HTL shows a favorable energy level with HTL for smooth hole injection from HTL to the QDs. The optimized device with QD:KHU-HTL(1 wt%) EML, the device achieved a maximum CE of 11.8 cd/A and EQE of 11.6%. The device also showed a half lifetime (LT₅₀) of 585 hours at an initial luminance of 1000 cd/m² and 36,191 hours at an initial luminance of 100 cd/m² which is relatively higher compared to without HTL doped EML devices. The single carrier HOD device analysis with QD:KHU-HTL(1 wt%) doped EML shows relatively faster current densities compared to the QD EML device. The reduction in electron injection and improvement in hole injection results in enhanced charge balance in the InP QLED device. This leads to high efficiency and a long device lifetime of InP QLED devices. We believe that this approach can be useful to attain high-performance QLEDs for display applications. Detailed results will be discussed at the presentation.

References:
[1] X. Dai, Y. Deng, X. Peng, Y. Jin, Adv. Mater., 2017, 29, 1607022.
[2] C. Y. Lee, N. N. Mude, R. Lampande, K. J. Eun, J. E. Yeom, H. S. Choi, S. H. Sohn, J. M. Yoo, J. H. Kwon, ACS Appl. Mater. Interfaces, 2019, 11, 36917−36924.
[3] K. Ding, H. Chen, L. Fan, B. Wang, Z. Huang, S. Zhuang, B. Hu, L. Wang, ACS Appl. Mater. Interfaces, 2017, 9, 20231−20238.

Quantum materials have recently exhibited many unique properties, which makes them extraordinary material for quantum optoelectronic applications. Among all QM, black phosphorus analogous tin(II) sulfide (SnS) has recently emerged as an attractive building block for photonic and optoelectronic devices due to its intrinsic anisotropic response.[1, 2] Two-dimensional SnS has shown to exhibit in-plane anisotropy in optical and electrical properties. However, the limitations in growing ultrasmall structures of SnS hinder the experimental exploration of anisotropic behavior in low dimension for emerging quantum applications. Here, we present an elegant approach of synthesizing highly crystalline nanometer-sized SnS sheets. Ultrasmall SnS exhibits two distinct valleys along armchair and zig-zag directions due to in-plane structural anisotropy like bulk crystal of SnS. We find that in ultrasmall SnS sheets, the bandgaps corresponding to two valleys are increased due to the quantum confinement effect. Moreover, the photoluminescence (PL) from SnS quantum dots (QDs) is excitation energy dependent.[3] Our spectroscopic studies infer that PL of SnS QDs originates from the two non-degenerate valleys. This work may open up an avenue to show the potential of SnS QDs for new functionalities in photonics and optoelectronics.
References
[1] A. S. Sarkar and E. Stratakis, Adv. Sci., 7, (2020) 2001655; F. Xia, H. Wang, J. C. M. Hwang, A. H. Castro Neto and Li Yang, Nat. Rev. Phys., 1 (2019) 306–317.
[2] A. S. Sarkar, A. Mushtaq, D. Kushavah, and S. K. Pal, npj 2D Mater. Appl., 4, (2020) 1-9.
[3] A. S. Sarkar, A. Kumari, Anchala, N. Nakka, R. Ray, E. Stratakis, and S. K. Pal, Appl. Phys. Lett. 119, (2021) 241902.

Observing the steady-state intraband transition of colloidal quantum dots has been a great challenge. For the last few years, there has been a rapid progress in realizing the steady-state intraband transition in colloidal quantum dots. The steady-state intraband transitions of self-doped Ag₂Se, HgSe, and β-HgS quantum dots have been successfully observed by varying the stoichiometry and surface dipole of the nanocrystal. Here, we present the interesting optical property of the PbSe quantum dots which appears near 10 μm without post-synthesis. We investigated the origin of the mid-IR absorption feature using FT-IR, X-ray photoelectron spectroscopy, and spectroelectrochemistry.

Quantum dot fabrication as ZnSe has been of great interest in recent decades due to its electrical, optical properties, and potential use in technology, biology, and medicine. Quantum dots (QDs) has shown extraordinary contributions due to its optical properties in optoelectrical devices, solar cells, photocatalytic activities, light-emitting diodes (LEDs), sensors, beam expanders among other contributions. Furthermore, previous studies have shown that ZnSe QDs show excellent photocatalytic activity in the degradation of contaminating dyes. Based on these considerations, the present research was focused on the synthesis, characterization, and modification of the QDs surface with different ligands. The presence of an organic cover (as thiols) on the QDs surface can affect its optical properties as fluorescence and absorbance. The objectives of this work were: I) synthesize nanoparticles with a size smaller than 10 nanometers (nm) in an aqueous medium with different thiols (L-glutathione, N-acetyl-L-cysteine, 3-mercaptopropionic acid (MPA) and thioglycolic acid (TGA), II) characterize zinc selenide (ZnSe) nanoparticles by different spectroscopy methods, and III) determine the point of zero charges IV) photodegradation of dyes in the presence of ZnSe quantum dots. Quantum Dots were synthesized in aqueous phase via reflux system. Precursors of Zinc (as zinc acetate dihydrate) and thiol compounds were mixed into a two-neck flask at temperature of 100 °C. As-synthesized nanocrystals were purified and precipitated with 2-propanol and resuspended in deionized water. Nanoparticles evidenced fluorescence peaks at 468 nm, 455 nm, 448 nm, and 397 nm for quantum dots covered with L-glutathione, N-acetyl-L-cysteine, 3-mercaptopropionic acid (MPA) and thioglycolic acid (TGA), respectively. Quantum dots evidenced remarkable optical properties and high stability in aqueous medium. Nanoparticles of ZnSe will be used to photodegrade organic contaminants.

Unusually high exciton binding energies (BEs), as much as ∼1 eV in monolayer transition-metal dichalcogenides, provide opportunities for exploring exotic and stable excitonic many-body effects. These include many-body neutral excitons, trions, biexcitons, and defect-induced
excitons at room temperature, rarely realized in bulk materials. Never-theless, the defect-induced trions correlated with charge screening have never been observed, and the corresponding BEs remain unknown. Here we report defect-induced A-trions and B-trions in monolayer tungsten disulfide (WS₂) viacarrier screening engineering with photogenerated carrier modulation, external doping, and substrate scattering. Defect- nduced trions strongly couple with inherent SiO₂ hole traps under high photocarrier densities and become more prominent in rhenium-doped WS₂. The absence of defect-induced trion peaks was confirmed using a trap-free hexagonal boron nitride substrate, regardless of power density . Moreover, many-body excitonic charge states and their BEs were compared via carrier screening engineering at room temperature. The highest BE was observed in the defect-induced A-trion state (∼214 meV), comparably higher than the trion (209 meV) and neutral exciton (174 meV), and further tuned by external photoinduced carrier density control. This investigation allows us to demonstrate defect-induced trion BE localization via spatial BE mapping in the monolayer WS₂ midflake regions distinctive from theflake edges.

Colloidal quantum dots emerged as an important class of functional materials for optoelectronic applications. II-VI quantum dots can be synthesized with exquisite control of size, shape and other characteristics. However, the solution synthesis of III-V quantum dots is mostly limited to indium pnictides which can be traced to a poor thermal stability of organic solvents at high temperatures required for nucleation and growth of non-indium III-V phases. Very few traditional solvents remain liquid above 400°C, and solvent or ligands decomposition becomes a serious problem at such temperatures. The use of inorganic salts as solvents eliminates this issue and offers new opportunities. As an example, we found that controllable synthesis of gallium and aluminum containing III-V quantum dots is very difficult if even possible using traditional organic solvents. However, various ternary and quaternary III-V quantum dots can be synthesized in molten inorganic salts.
I will also discuss the advances in the surface chemistry of semiconductor nanostructures. By making surface ligands photochemically or thermally cleavable, we introduced a general approach for photoresist-free, direct optical lithography of functional inorganic nanomaterials (DOLFIN). Examples of patterned materials include quantum dots, metals, oxides, and magnetic materials. No organic impurities are present in the patterned layers, which helps achieve good electronic and optical properties. The ability to directly pattern all-inorganic layers using a light exposure dose comparable to that of organic photoresists opens up a host of new opportunities for additive nanomanufacturing.

Colloidal quantum dots (QDs) have been known to be the best candidates for photon emissive materials owing to their unique optical properties including high color purity and quantum efficiency. Cd-based QDs like CdSe, CdS, and CdTe have been extensively studied and their synthesis and application methods are very well developed. However, their potential harmful effects on health and the environment prohibit the applications to commerical products. InP QDs have been considered as the best alternative owing to their band gaps corresponding to visible light as well as their relatively low toxicity, despite the high tendency of oxidation and the small defect tolerance originating from relatively high covalent character. In this talk, we will present InP-based core/shell-type QDs showing almost unity photoluminescence quantum efficiency and long-term stability on high power blue irradiation. Based on this superior optical properties, the QDs could be applied for the color conversion pixels on blue OLED display. Structural and spectroscopic analysis about the In-based QDs will be introduced as well.

Semiconductor colloidal quantum dots (CQDs) have been widely used in many applications such as optoelectronic and energy-harvesting technology. The intense research on CQDs is triggered by the ease of access to modify both of electrical and optical properties with numerous approaches on pre- and post-synthesis processes. Efforts have been made to optimize the performance of CQDs-based devices, such as controlling the stoichiometry of the QDs as well as the ligand-related surface doping. Recently, it was shown that the performance of charge carrier transport on the CQDs solid assembly is strongly affected by the degrees of dot-to-dot connection and the corresponding structural factors. Nevertheless, obtaining disorder-free CQDs assembly still remains a challenge. Conventionally, ligand-exchange process (long- to short-surface-ligand) must be introduced in order to achieve electronically coupled assembly. Another possible approach is by pursuing the formation of the superstructure of CQDs assembly through the manipulation of the self-assembly process. This process shows an intriguing ability to organize CQD into quasi-2D superstructure with oriented attachments of its facets, leading to a promising way to obtain a structure that is able to support the demonstration of remarkable performance of CQDs-based technology.

Here we demonstrate facile control of lead sulfide (PbS) CQD assemblies to form large-scale epitaxially-connected superlattice with oriented attachment, in which we successfully elucidate their electronic transport. We varied the size of the QDs that become the building block of the assemblies. The superlattice assemblies were prepared by a modified liquid/air interfacial assembly. The attachment and orientational arrangement of QDs were controlled chemically by choosing the proper solvent to support the ligand removal on the [100] facets of the QDs and to further continue the ligand stripping on [111] gently using an amine-based stripping agent. Using combinations of transmission electron microscopy (TEM/HR-TEM), grazing-incidence small-angle x-ray scattering (GISAXS), and grazing-incidence wide-angle x-ray scattering (GIWAXS) measurements, we can observe the orderings in the length scale of the superlattice and determine the orderings in the atomic details to judge the specific orientation and the specific atomic connections on the QD facets. Having large-area strongly ordered 2D superlattices, the electronic transport properties of QDs assemblies were evaluated by using ionic-liquid-gating field-effect-transistors (FETs). Probing by electric-double-layer (EDL)-induced-gating, the size-dependent bandgap of the QDs assemblies can be observed electronically. This electronic transport of the epitaxially-connected superlattices demonstrates high conductivity while the quantum confinement nature of the QDs is still maintained. We will discuss the comparison of the QD size influence on charge carrier transport processes in the epitaxially-connected QD superlattices and in the conventional ligand-bridged QD assemblies. These two different systems showed distinct electronic transport behavior when the size of the QD building blocks are varied, suggesting different key factors in determining the charge carrier transport mechanism. The understanding of charge carrier transport in the epitaxially-connected superlattice of the quantum-confined QDs will reveal the key indicators to significantly enhance the performance of QDs-based optoelectronic and energy harvesting devices for practical applications that rely on efficient charge-carrier transport.
References:
[1] R. D. Septianto, S. Z. Bisri, et al. NPG Asia Materials. 12, 33 (2020)
[2] L. Liu, S. Z. Bisri, et al. Nanoscale 11, 20467 (2019)
[3] L. Liu, S. Z. Bisri, et al. Nanoscale 13, 14001 (2021)
[4] R. D. Septianto, S. Z. Bisri, et al. in preparation
[5] N. Yazdani, V. Wood, et al. Nature Comm. 11, 2852 (2020)
[6] T. Chen, B. I. Shklovskii, et al. Nature Materials 15, 299 (2016)

Colloidal nanoplatelets show some of the most promising optical gain characteristics of any materials. Using a modified quantum fountain design, thresholdless intraband optical gain at near-infrared wavelengths is demonstrated in using colloidal nanoplatelets. This is the first demonstration of unipolar gain in a colloidal system, requiring only electrons in this case. Stimulated emission wavelength depends on the thickness of the samples and can be tuned to technologically important wavelengths such as 1.3 and 1.5 μm. The modal gain, gain bandwidth, and gain lifetime of this intraband gain may be manipulated using excitation energy and temperature. Unlike bipolar laser media which competes with non-radiative Auger recombination, intraband stimulated emission must compete with intraband relaxation via longitudinal optical and surface phonons.

In this talk, I will discuss our efforts to use a hybrid organic-inorganic platform consisting of nanocrystals functionalized with dye molecules to produce circularly polarized luminescence. Over the past 5 years, we have experimentally shown that triplet excitons can be efficiently transferred between photoexcited quantum dots and surface-bound conjugated organic molecules. The idea is to harness the phosphorescence or photoluminescence from chiral ligands that are electronically excited by energy transfer from quantum dots. Excitons in these quantum dots can photogenerated or electrically pumped, depending on the device architecture. Starting with photoexcited CdS QDs functionalized with chiral naphthalene ligands, we quantify chiral induced spin selectivity using steady-state and time resolved circular dichroism/ circularly polarized emission. Ultimately, the goal of this research is a solution processed, optoelectronic thin film that electrically generates chiral emission.

Assemblies of colloidal semiconductor nanocrystals (NCs) in the form of thin solid films leverage the size-dependent quantum confinement properties and the wet chemical methods, vital for the development of the emerging solution-processable electronics, photonics, and optoelectronics technologies. The ability to control the charge carrier transport in the colloidal NC assemblies is fundamental for altering their electronic and optical properties for the desired applications. While surface doping and ligand manipulations have been very effective, they generally suffer from the sample-to-sample variability and insufficient long-term retention of the surface stoichiometries due to environmental effects. Wherever possible and applicable, the formation of a more stable core@shell morphology wherein shell acts as a dopant and carrier-regulating region is preferred. For many applications in devices where charge carrier transport functionality is one of the most prominent, the role of shells in the core@shell NCs is still not well understood. Here we demonstrate exclusive electron transport in the assemblies of core@shell PbTe@PbS colloidal NCs. In contrast to the ambipolar characteristics demonstrated by many narrow bandgap NCs, the core@shell NCs exhibit exclusive n-type transport; i.e., drastically suppressed contribution of holes to the overall transport. The PbS shell that forms type-II heterojunction assists the selective carrier transport by heavy doping of electrons into the PbTe-core conduction level and simultaneously strongly localizes the holes within the NC core valence level. The demonstration of exclusive electron transport in this core@shell NCs is beneficial for their applications in devices or device components where ambipolar characteristics of the materials should be strongly suppressed, such as in thermoelectric and electron transporting layers in light-emitting diodes or solar cells. The stability of the electron transport in these core@shell NCs and the tuning of the heterojunction formation by the shell thickness control and modification will be also discussed.

References:
[1] R. Miranti, D. Shin, R. D. Septianto, M. Ibáñez, M. V Kovalenko, N. Matsushita, Y. Iwasa, S. Z. Bisri, ACS Nano 14, 3242 (2020)
[2] R. Miranti, R. D. Septianto, M. Ibáñez, M. V Kovalenko, N. Matsushita, Y. Iwasa, S. Z. Bisri, Appl. Phys. Lett. 17, 173101 (2020)
[3] R. Miranti, S. Z. Bisri et al. in preparation

Fluorescent microbeads are widely used for applications in life sciences and medical diagnosis. The spectral contrast and sharpness of photoluminescence are critical in the utilities of microbeads for imaging and multiplexing. Recently, there has been growing interests in using microbeads in a lasing or stimulated emission regime to enhance spectral brightness and multiplexing capability. Here, we demonstrate quantum dot microbeads capable of generating single-peak laser emission with a sub-nanometer linewidth.^[1] The microbeads are made of quantum dots that are tightly packed and crosslinked via ligand exchange for high optical gain and refractive index as well as material stability. Bright single-mode lasing with no photobleaching is achieved with particle diameters as small as 1.5 μm in the air. Sub-nm lasing emission is maintained even inside high-index surroundings, such as organic solvents and biological tissues. The feasibility of intracellular tagging and multi-color imaging in vivo is demonstrated. Multicolor imaging with proposed microbeads shows greatly enhanced spectral density and color-multiplexing capability compared to conventional fluorescence imaging.

[1] K.-H. Kim et al. Adv. Funct. Mater. 2021, 2103413

Two-dimension (2D) transition-metal chalcogenides (TMCs) have recently provided a rich source of research opportunity, revealing interesting physical phenomena including quantum-spin Hall effect (QSH), valley polarization, 2D superconductivity. Among 2D TMCs, novel 2D semiconductors play an increasingly important role in modern nanoelectronics and optoelectronics.

I will introduce the potential of 2D materials to build a sustainable world in terms of novel electronic and optoelectronic devices. Various novel 2D TMC semicondcutors have been fabricated at the material level based on chemical vapor deposition and solid reaction methods. For example, air-stable 2D semiconductor PdPS and PdPSe with a tailored puckered structure is successfully designed and synthesized [1, 2]. At the device level, based on the 2D semiconductor and ferroelectric materials, we have demonstrated the negative capacitance field-effect transistors and memory, which may dramatically reduce energy consumption [3, 4]. I will also discuss our recent progress on machine learning guided materials synthesis and the novel design of neural networks hardware based on 2D materials [5, 6].

References
Duan R. H., et al. 2D Cairo Pentagonal PdPS: Air-Stable Anisotropic Ternary Semiconductor with High Optoelectronic Performance, Advanced Functional Materials 2113255, 2022.
Duan R. H., et al. PdPSe: Component-Fusion-Based Topology Designer of Two-Dimensional Semiconductor, Advanced Functional Materials, 31, 2102943, 2021.
Wang X. W., et al. Van der Waals engineering of ferroelectric heterostructures for long-retention memory, Nature Communications 12, 1109, 2021.
Wang X. W., et al. Van der Waals negative capacitance transistors, Nature Communications 10, 3037, 2019.
Tang B.J., et al. Machine learning-guided synthesis of advanced inorganic materials, Materials Today 41, 72, 2020.
Chen J. G. et al. Mimicking Neuroplasticity via Ion Migration in van der Waals Layered Copper Indium Thiophosphate, Advanced Materials 2104676, 2021.

Two-dimensional layered materials (2D-LMs) materials have outstanding physical, chemical and thermal properties that make them attractive for the fabrication of solid-state micro/nano-electronic devices and circuits. However, synthesizing high-quality 2D-LMs at the wafer scale is difficult, and integrating them in semiconductor production lines brings associated multiple challenges. Nevertheless, in the past few years substantial progress has been achieved and leading companies like TSMC, Samsung and IMEC have started to work more intensively on the fabrication of devices using 2D-LMs. In this talk, I will discuss the state-of-the-art on micro/nano-electronic devices made (entirely or partially) of 2D-LMs, as well as their integration in CMOS chips. I will present the most sophisticated prototypes developed so far, with special emphasis on devices that employ hexagonal boron nitride, the only 2D-LM with an enough high band gap to be employed as dielectric. I will also discuss the main technological challenges to face in the next years and provide some recommendations on how to solve them.

Organic-inorganic hybrid perovskites (OIHP) recently have received considerable attention for their applications in a wide range of optoelectronic devices due to their unique optical and electronic properties and their potential to be fabricated using low-cost manufacturing methods. Research of their use in field-effect transistors (FET) has received less attention, struggled in particularly to obtain room temperature mobility due to ion migration. Two-dimensional (2D) halide perovskites have displayed natural advantages in suppressing ion movement and improving the ambient stability of their devices. In this talk, I will present a molecular approach to the synthesis of stable organic-inorganic hybrid 2D Sn-based perovskites for FETs through incorporating linear π-conjugated oligothiophene ligands. The conjugated ligands can facilitate the formation of micrometer-size large grains, improve charge injections, and stabilize the inorganic perovskite layers. By extending the π-conjugation and increasing the planarity of the semiconducting ligand, nucleation density can be decreased by more than 5 orders of magnitude. Therefore, the thin-film nucleation and growth kinetics in 2D halide perovskites were well controlled to form thin films with highly ordered crystalline structures and extremely large grain sizes. Then, high-performance field-effect transistors with hole mobility approaching 10 cm²V^-1s^-1 and ON/OFF current ratios of ~10⁶ were demonstrated on wafer-scale 2D perovskite thin films. It was found that the large conjugated organic layers not only serve as thick and dense barriers for moisture and oxygen, but also increase the crystal formation energy via strong intermolecular interactions. These molecularly engineered 2D semiconductors are promising candidates for use in high-performance electronics and optoelectronics.

The understanding of ultrafast carrier dynamics is the foundation of quantum materials applications in renewable energy, nanotechnology, and quantum information and technology. To reveal the ultrafast dynamics, the majority research groups rely on classic ultrafast optical spectroscopies such as pump-probe transient absorption spectroscopy and time-resolved photoluminescence. Although these non-contact photon-in and photon-out approaches have made revolutionary discoveries, which have been demonstrated by our pioneers’ work in Femtochemistry (Zewail, 1999), super resolution microscopy (Hell, 2014), and chirped pulsed amplification (Strickland, 2018), they lack an in-situ feature for characterizing building block devices such as solar cells, LEDs, photoconductors, transistors, etc..
In this talk, I will highlight the ultrafast photocurrent spectroscopy that we have developed to bridge the gap between classic optical spectroscopies and carrier dynamics in-situ devices. In particular, I will focus on quantum materials including perovskite nanocrystals and 2D layer transition metal dichalcogenide (TMDC). Because of their unique quantum confinement effect, leading to the novel phenomena such as hot carrier and exciton condensation, the understanding of ultrafast dynamics may lead to next generation hot carrier solar cells and room temperature superconductors.