Symposium EP10 : Optoelectronics of Two-Dimensional (2D) Materials

The electronic properties of topological insulators – narrow band-gap semiconductors that exhibit insulating bulk and semimetallic Dirac surface states – have been the subject of intense study over the past several years. The optical and optoelectronic behavior of these materials, however, remain widely uncharacterized. We report here experiments that demonstrate electronic control of the optical properties of 5-20 nm thick (Bi_1-xSb_x)₂Te₃ films grown by Van der Waals epitaxy, both as-grown on strontium titanate substrates, and after epitaxial lift off and transfer to silicon dioxide on silicon.

It has previously been shown that electrostatic gating can be used to tune the Fermi level in the Dirac semimetal graphene, modifying interband transitions and free carrier absorption. We find strong optical absorption modulation for (Bi_1-xSb_x)₂Te₃ films by electrostatic gating, and argue that this modulation is due to bulk states, not topological surface states. In particular, we see 2 and 5 relative percent modulation of transmittance and reflectance, respectively, in the 3-10 micron wavelength range for films transferred to silicon dioxide on silicon substrates. We see similar behavior in as-grown films via ellipsometry. The spectral locations of the modulation features, combined with electrical measurements, indicate that we are modifying bulk carrier interband transitions and free carrier absorption by gate-induced modulation of the Fermi level in p-type (Bi_1-xSb_x)₂Te₃ films. We also report a novel epitaxial lift off process for topological insulator films grown on strontium titanate – which is a preferred growth substrate for TI materials, but which is non-ideal for infrared optical characterization – to transfer films to other substrates. These results demonstrate the gate tuning of semiconductor interband transitions, and provide a characterization of the bulk optical response of topological insulators; transport properties of these films will also be discussed.

The ability to efficiently detect light over a broad spectral range is central to various technology applications such as imaging, sensing, spectroscopy and communication etc. Graphene is among one of the most promising candidate materials for ultra-broadband photodetectors, as its absorption spectrum covers the entire ultraviolet to far-infrared range due to its unique gapless linear band structure. However, the photoresponsivity of graphene-based photodetectors has so far been limited due to the small optical absorption coming from its monolayer structure nature. Here, we report an ultra-broadband photodetector design based on graphene heterostructures. The detector is a phototransistor utilizing the tunneling barrier formed by the heterojunction interface. Under optical illumination, photo-excited hot carriers generated in the absorption layer tunnel into channel layer, leading to a charge build-up in absorption layer and a strong photo-gating effect on the channel conductance. The single pixel detector demonstrates room temperature photodetection from visible to the near-infrared range, with near-infrared responsivity higher than 1 A/W. In addition, infrared imaging using a 1000-pixel detector array will also be discussed. These results address the key challenges for broadband infrared imaging array and showcase the promise of graphene-based optoelectronic applications.

The emergence of a wide variety of two-dimensional (2D) materials has created new opportunities for device designs and applications. In particular, the availability of semiconducting transition metal dichalcogenides,¹ in addition to semi-metallic graphene and insulating boron nitride, has enabled the fabrication of ‘all 2D’ van der Waals heterostructure devices. However, the concept of van der Waals heterostructures^{1, 2} has the potential to be significantly broadened beyond layered solids. This principle has been exploited to fabricate heterojunctions between conventional silicon and 2D materials³ as well as carbon nanotubes with 2D materials⁴ and amorphous oxide semiconductors.⁵ Similarly, molecular and polymeric organic solids,⁶ whose surface atoms possess saturated bonds, have the potential to interact via van der Waals forces and thus offer an alternative for scalable integration with 2D materials. Despite this potential, the integration of organic semiconductors with 2D materials for optoelectronic devices has thus far been limited. Here, we demonstrate the integration of an organic small molecule p-type semiconductor, pentacene, with a 2D n-type semiconductor, MoS₂. The resulting p-n heterojunction diode is gate-tunable and shows asymmetric control over the anti-ambipolar transfer characteristic. The operating principles and band profiles of this system are characterized by direct charge transport measurements, scanning photocurrent microscopy, electrostatic force microscopy, and finite element modeling. This comprehensive experimental and computational study reveals that pentacene forms a type-II heterojunction with MoS₂ exhibiting a photovoltaic effect, thereby demonstrating n-type MoS₂ as a candidate non-fullerene acceptor in organic photovoltaics.

References:
1. Jariwala D., Sangwan V.K., Lauhon L.J., Marks T.J., Hersam M.C. Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides. ACS Nano 8, 1102–1120 (2014).

2. Grigorieva I.V., Geim A.K. Van der Waals heterostructures. Nature 499, 419-425 (2013).

3. Yang H., Heo J., Park S., Song H.J., Seo D.H., Byun K.-E., et al. Graphene Barristor, A Triode Device with a Gate-Controlled Schottky Barrier. Science 336, 1140-1143 (2012).

4. Jariwala D., Sangwan V.K., Wu C.-C., Prabhumirashi P.L., Geier M.L., Marks T.J., et al. Gate-Tunable Carbon Nanotube–MoS₂ Heterojunction p-n Diode. Proc. Nat. Acad. Sci. USA 110, 18076–18080 (2013).

5. Jariwala D., Sangwan V.K., Seo J.-W.T., Xu W., Smith J., Kim C.H., et al. Large-Area, Low-Voltage, Antiambipolar Heterojunctions from Solution-Processed Semiconductors. Nano Lett. 15, 416-421 (2015).

6. Marks T.J. Materials for organic and hybrid inorganic/organic electronics. MRS Bull. 35, 1018-1027 (2010).

The optoelectronic response of two-dimensional (2D) crystals, such as graphene and transition metal dichalcogenides (TMDs), is currently subject to intensive investigations. Owing to its broadband absorption, gapless character and ultrafast carrier dynamics, graphene is a promising material for nano-photonics and high-speed photodetectors [1], whereas TMDs have emerged as potential candidates for sensitive photodetection thanks to their enhanced photon absorption. Vertically assembling these crystals in so-called van der Waal heterostructures allows the creation of novel and versatile optoelectronic devices that combine the complementary properties of their constituent materials.

Here we present a various new device capabilities, varying from nano-photonic devices to ultra-fast and broadband electrical detectors [1-5]. We applied femtosecond time-resolved photocurrent measurements on 2d material heterostructures, which probes the transit of photoexcited charges across the photoactive TMD layer – and thus current generation – directly in the time domain. Remerkably fast photoresponse of 5 ps is observed.
In adition, we show that graphene-WSe2-graphene heterostructure devices offer this possibility through the photo-thermionic (PTI) effect: the absorbed photon energy in graphene is efficiently transferred to the electron bath, leading to a thermalized ‘hot’ carrier distribution. Carriers with energy higher than the Schottky barrier between graphene and WSe2 can be emitted over the barrier, thus creating photocurrent. We experimentally demonstrate that the PTI effect enables detection of sub-bandgap photons, while being size- scalable, electrically tunable, broadband and ultrafast.

References:
[1] Photodetectors based on graphene, other two-dimensional materials and hybrid systems
F. H. L. Koppens, T. Mueller, Ph. Avouris, A. C. Ferrari, M. S. Vitiello, M. Polini
Nature Nanotechnol. 9, 780-793 (2014)
[2] High-Responsivity Graphene–Boron Nitride Photodetector and Autocorrelator in a Silicon Photonic Integrated Circuit.
Ren-Jye Shiue, Yuanda Gao, Yifei Wang, Cheng Peng, Alexander D Robertson, Dmitri K Efetov, Solomon Assefa, Frank HL Koppens, James Hone, Dirk Englund
Nano Letters, 15 (11), 2015.
[3] Picosecond photoresponse in van der Waals heterostructures
M. Massicotte, P. Schmidt, F. Vialla, K. G. Schädler, A. Reserbat-Plantey, K. Watanabe, T. Taniguchi, K. J. Tielrooij and F. H. L. Koppens
Nature Nanotechnology 11.1 (2016)
[4] Hot-carrier photocurrent effects at graphene–metal interfaces
K. J. Tielrooij, M. Massicotte, L. Piatkowski, A. Woessner, Q. Ma, P. Jarillo-Herrero, N. F. van Hulst, F. H. L. Koppens
J. Phys.: Condens. Matter 27, 164207 (2015)
[5] Generation of photovoltage in graphene on a femtosecond timescale through efficient carrier heating
K. J. Tielrooij, L. Piatkowski, M. Massicotte, A. Woessner, Q. Ma, Y. Lee, K. S. Myhro, C. N. Lau, P. Jarillo-Herrero, N. F. van Hulst, and F. H. L. Koppens
Nature Nanotechnology 10, 437-443 (2015)

The enhancement of spontaneous emission rates is an important goal of the nanophotonics community for applications such as lasers, single photon sources, ultrabright LEDs, and ultrafast signal processing. Plasmonic structures are promising candidates for accomplishing this goal because they strongly confine light, thus increasing the local density of optical states, LDOS, leading to enhanced spontaneous emission. We demonstrate the first reported increase in spontaneous emission rate by means of plasmonic coupling of graphene nanoribbons to spontaneous emitters, using Er as an active infrared luminescent center. By electrostatically tuning the plasmon resonance to that of the erbium 2.94µm ⁴I_11/2 → ⁴I_13/2 transition, we show an enhancement in luminescence for the 1550nm ⁴I_13/2 → ⁴I_15/2 transition. We realize this experimentally using graphene nanoresonators patterned with widths between 15nm and 25nm on top of 20nm to 200nm thick layers of erbium-dopedY₂O₃. We add an ion gel to aid in doping of the graphene by charge accumulation. The sample is optically pumped at a wavelength of 532nm and its emission is measured near 1550nm as a function of voltage applied to the ion gel. We improve the luminescence enhancement of adding a plasmonically-resonant 25nm graphene nanoribbon by 75% compared to undoped nanoribbons. As the plasmon only interacts with the top few tens of nanometers of material and the effect we observe is an indirect measurement of the transition, this increase in luminescence implies a strong increase in the spontaneous transition rate of the 2.94µm transition. We therefore show an electrically-tunable Purcell-enhanced plasmonic emitter that can increase luminescence at 1550nm.
Previous work in metallic nanoparticles has demonstrated spontaneous emission rate enhancements exceeding 1,000¹. We also discuss the outlook for upper limits to graphene plasmon-enhanced Er spontaneous emission. Prior work has shown that graphene nanoresonators support highly confined plasmonic modes in the mid-infrared with a LDOS of 10⁶ times greater than that of free space². It has been predicted that this extremely large LDOS can proportionately enhance the emission of rare-earth emitters, via emission to a plasmonic mode of gated graphene nanoresonators, thus allowing widely tunable control of the Purcell enhancement³.
[1] T. Hoang, G. Akselrod, C. Argyropoulos, J. Huang, D. Smith, and M. Mikkelsen, Ultrafast spontaneous emission source using plasmonic nanoantennas. Nature Communications. 2015, 6 (7788).
[2] V. Brar, M. Jang, M. Sherrott, J. Lopez, and H. Atwater. Highly Confined Tunable Mid-Infrared Plasmonics in Graphene Nanoresonators. Nano Letters. 2013, 13 (6), 2541-2547.
[3]. K. Tielrooij, et al. Electrical control of optical emitter relaxation pathways enabled by graphene. Nature Physics. 2015, 11, 281-287.

In this work, we demonstrate coherent perfect absorption of terahertz radiation in electrolyte gated graphene. We developed a new electrically tunable terahertz cavity using graphene and gold electrodes, filled with a room-temperature ionic liquid electrolyte. The conductivity of the graphene electrode, thus the terahertz reflectivity is controlled by a voltage applied between the electrodes. We were able to modulate the resonant reflectivity by 99% with a bias voltage of 2V. We anticipate that the gate-tunable coherent perfect absorption would lead to efficient active terahertz devices such as modulators and detectors.

Two-Dimensional layered heterostructures open up great prospects in photodetector applications¹. It is a promising route to decouple the charge photogeneration from their transport by mean of photogating, as in the case of hybrid graphene-2D semiconductors^2,3. In this context, the charge transfer control between the light absorber layer and the transport layer is of crucial importance. In order to evidence and overcome the restricting parameters for charge transfer, we build a model system by hybridizing colloidal 2D CdSe nanoplatelets⁴ with epitaxial graphene⁵. This strategy bypasses the limited mobility of nanocrystals films originating from the hopping transport⁶.

The charge transfer is investigated by building an electrolyte gated photo-transistor. We show that the large semiconductor exciton binding energy originating from the 2D character of the material prevents an efficient charge transfer to the graphene channel. This bottleneck can be overcome by growing a heterostructure at the nanocrystal level, allowing us to control the magnitude and the direction - electrons or holes - of the charge transfer to graphene. We eventually show that functionalizing graphene by nanocrystals only leads to a limited change in the magnitude of the 1/f noise.

(1) Buscema, M.; Island, J. O.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A. Chem. Soc. Rev. 2015, 44 (11), 3691–3718.
(2) Konstantatos, G.; Badioli, M.; Gaudreau, L.; Osmond, J.; Bernechea, M.; de Arquer, F. P. G.; Gatti, F.; Koppens, F. H. L. Nat. Nanotechnol. 2012, 7 (6), 363–368.
(3) Sun, Z.; Liu, Z.; Li, J.; Tai, G.; Lau, S.-P.; Yan, F. Adv. Mater. 2012, 24 (43), 5878–5883.
(4) Ithurria, S.; Tessier, M. D.; Mahler, B.; Lobo, R. P. S. M.; Dubertret, B.; Efros, A. L. Nat. Mater. 2011, 10 (12), 936–941.
(5) Robin, A.; Lhuillier, E.; Xu, X. Z.; Ithurria, S.; Aubin, H.; Ouerghi, A.; Dubertret, B. Submitted.
(6) Lhuillier, E.; Robin, A.; Ithurria, S.; Aubin, H.; Dubertret, B. Nano Lett. 2014, 14 (5), 2715–2719.

Transition-metal dichalcogenides (TMDs) are two dimensional (2D) nanomaterial with the common formula MX₂, where M is a transition metal element from group IV-VII (M = Mo, W, Nb, Re, and so on) while X is a chalcogene element (X = S, Se, Te). In general, M atoms are sandwiched between X atoms to form a single layer, and each layer can be stacked together via van der Waals forces, which make 2D TMDs easily cleaved by scotch tape or other similar techniques. Among TMD families with ultra-thin layers, molybdenum disulfide (MoS₂) and tungsten diselenide (WSe₂) are well-known semiconductors with their bandgaps of more than 1 eV; their band gap increases to 1.6~1.8 eV for monolayer and band properties changes from indirect to direct type. As one of quite recent 2D materials, molybdenum ditelluride (α-MoTe₂) has also been attracting attention due to its optical and electrical properties. Monolayer a-MoTe₂ exhibits a direct optical bandgap of 1.10 eV, while its bulk form becomes an indirect semiconductor with the band gap of 0.88~1.0 eV. Interestingly, it is reported that MoTe₂ shows structural and electronic phase transition. The structural phase transition from hexagonal (2H) phase to monoclinic (distorted octahedral or 1T ) phase is reversible at a high temperature.
According to a literature, few-layered a-MoTe₂ field effect transistors (FETs) showed ambipolar type conduction with somewhat low mobilities of 0.2 ~ 3.7 cm²V^-1s^-1. But another report displayed a p-channel MoTe₂ nanoflake FET of quite enhanced mobility of 10~30 cm²V^-1s^-1 using thermal annealing for contact. MoTe₂-based homo- or hetero-junction p-n diode has not been reported yet although p-n diode is one of the basic building blocks for electronics and optoelectronics. In fact, forming two different types of conduction in the same nanoflake might not be easy. So, heterojunction p-n diode studies have always been preferred but limited to n-MoS₂/p-Si bulk, n-MoS₂/p- WSe₂ and n-MoS₂/p-BP (black phosphorous) systems, to the best of our knowledge.
In the present study, we attempted van der Waals heterojunction p-n diode fabrication by direct imprinting technique transferring p-type α-MoTe₂ onto n-type MoS₂ nanoflake, since in this way we would achieve Mo-based dichalcogene 2D p-n diode which appears novel in view of using the same transition metal for both p- and n-type. Pt electrode appeared very good for ohmic contact with p-type α-MoTe₂ even without thermal annealing. Our few-layered a-MoTe₂/MoS₂ p-n diode demonstrates low voltage operation at 5 V and high reasonable ON/OFF current ratio of 10³~4×10³ (~4×10³ on SiO₂/Si and ~10³ on glass substrate) along with good ideality factors of 1.06~1.34. Kilohertz fast dynamic rectification was achieved in the dark while dynamic photo-voltaic switching was also exhibited under red, green, blue LEDs, and 800 nm infrared laser. We regard that our new 2D p-n diode is quite promising in the prospects of nano-electronics and optoelectronics.

This study presents using a nanocomposite comprising of a cationic conjugated polyelectrolyte (CPE), Poly[(2,5-bis(2-(N,N-diethylammonium bromide)ethoxy)-1,4- phenylene)-alt-1,4-phenylene] or (PPPNEt2.HBr), with graphene oxide (GO) as a new hole
injection layer (HIL) for organic light emitting diodes. It is demonstrated that using the designed ionically functionalized water soluble conjugated polymers instead of polyethylene dioxythiophene: polystyrene sulfonate (PEDOT:PSS) is a promising approach to overcome
strong acidic nature of PEDOT:PSS besides excluding its non-conductive PSS part. As the other aspiration of this work, we introduce a good partner for dissolving and spin-casting of GO as a simple and economic technique to use the hole conductive and electron blocking nature of GO in hole injection portion of assembled devices. Using this new binary blend showed enhanced charge carrier mobility, good electroluminescent and J-V characteristics in comparison with the conventional devices. Such improvement is interpreted with induced ion space charge of HIL at the interface and resulting electric field screening effect due to ion migration.

Researching and developing of high efficiency and inexpensive alternatives to precious metals Pt counter electrode material is critical for promoting the dye-sensitized solar cell (DSSCs) of large-scale commercialization. First by studying the mechanism of iodine reduction reaction (IRR) occurring on the counter electrode, concluded that elementary reactions (III) is rate-determining steps to the overall electro-catalytic activities. So IRR rate mainly depends on the ionization energy of the electro-catalyst. And by screening the volcanic figures, amino-functionalized reduced graphene oxide (EFG) is preferably to be used as counter electrode material owing to the lower ionization energy and optimal iodine binding energy. Then under mild experimental conditions we synthesized EFG by ethylene diamine nucleophilic epoxy functional groups surface in graphene oxide and assembled into DSSCs as a counter electrode. Benefiting from lower ionization energy and the unique structure, EFG has a high power conversion efficiency (PCE) close to that based traditional Pt counter electrode, which will transfer electrons rapidly from external circuit to the I₃⁻, thus reducing the charge transfer resistance (R_ct) and improving the photovoltaic performance. The present work has a guiding significance for theoretical design of high-efficiency carbon electrodes in energy conversion/storage devices.

Hot filament chemical vapor deposition (HFCVD) reactor shows a great potential of being implemented in the high quality synthesis of carbon nanomaterials. Herein, we report the synthesis of large area single crystal monolayer graphene islands of area up to 100 µm on hydrogen terminated germanium (110) surface by using methane gas as a carbon precursor in HFCVD reactor. As synthesized graphene on germanium substrate was primarily characterized by Raman spectroscopy which shows narrow G peak at 1588 cm^-1 with a full wave at half maximum (FWHM) of about 13 cm^-1 and 2D peak at 2682 cm^-1 with a FWHM of about 34 cm^-1. Field emission scanning electron microscopy image shows the growth of multiple single crystal islands on the germanium surface. By optimizing the values of gas flow rates and deposition time, we were able to control the size of the graphene islands on the germanium substrate. The anisotropic twofold symmetry of the germanium (110) surface allows unidirectional alignment of multiple graphene islands so we believe that such islands merge to produce a single crystal large area graphene under an optimized condition.

GaN-based vertical structure LED (VLED) power chips show several advantages including good current spreading, high light extraction efficiency and reduced forward voltage. However, the GaN VLED power chips also suffer from loss of light absorption by n-electrode and current spreading limitation in lateral direction. These problems were previously minimized by p-GaN plasma etching, p-GaN ion bombardment and silicon dioxide (SiO₂) layer deposition on p-GaN of the GaN VLED structure. Nevertheless, the above-mentioned approaches also cause certain drawbacks including reverse leakage current (I_R) increment and poor adhesion of SiO₂ layer on reflective mirror layer.
In this paper, instead of direct deposition of a SiO₂ layer on top of the p-GaN of a GaN VLED structure, a SiO₂ layer was deposited right underneath the reflective mirror layer of a GaN VLED structure. In this way, SiO₂ layer was employed to serve as a current distribution layer (CDL) underneath the n-electrode of a GaN VLED power chip so as to limit current flowing in vertical direction and to enable current distribution in lateral direction of the GaN VLED power chip. In this case, SiO₂ CDLs having various thicknesses and widths were processed on GaN VLED structures by employing plasma enhanced chemical vapor deposition (PECVD) and lithography with a typical power chip size of 55 x 55 mil². It was found that the best GaN-based VLED performance occurred in an SiO₂ CDL with thickness and width being 500 Å and 50 µm, respectively. Under an injection current of 350 mA, the forward voltage (V_F) and light output power (L_OP) of the best GaN VLED were 2.97 V and 672 mW, respectively, with its wall-plug efficiency (WPE) being 64.6% which shows an 8.7% enhancement in WPE compared to the WPE of a GaN VLED power chip having the same structure without CDL.
Key words : GaN vertical structure light-emitting diode (VLED), current distribution layer, LED power chip

Multifarious ZnO morphologies (Hollow spheres and Bucky bowls) and Graphene Oxide/ZnO hybrid structures (nanorod and triangle shaped) were fabricated through a facile hydrothermal method. The complex structural evolution of the ZnO and GO-ZnO hybrid structures was carefully analyzed through both experimental and analytical means (deformation models), we controlled the morphology of the ZnO and verified their growth mechanism. Dislocation driven growth is a favored mechanism and is also common effect for GO-ZnO nanorods and GO-ZnO triangles. The oxygen vacancies (V_O) created by dislocation growth in GO-ZnO strongly influence the structure and photoelectrochemical (PEC) performance of the ZnO. The GO-ZnO triangle-shaped composites exhibit the maximum overall water splitting photocurrent density of ~1.517 mA/cm^-2 vs RHE with the highest enhancement in the Incident Photon to Current conversion Efficiency (IPCE) of 10.41% under UV light irradiation. In addition, the PEC performance of these photoelectrodes was enhanced by increasing the oxygen defects to some extent and then decreased with the further addition oxygen defects, which indicates that an optimum amount of oxygen vacancies will improve the PEC efficiency of the GO-ZnO composites.

Two-dimensional layered semiconductors such as MoS₂ and WSe₂ have attracted considerable interest in recent times as semiconductor after Si. Much like the traditional semiconductor technique, controlling the space distribution of composition and electronic structure of two dimensional semiconductor material is essential to copnstruct all modern electronic and optoelectronic devices, including transistors, p–n diodes, photovoltaic/photodetection devices, light-emitting diodes and laser diodes. With a relatively small lattice mismatch (∼4%) between MoS₂ and MoSe₂ or WS₂ and WSe₂, it is possible to produce coherent MoS₂–MoSe₂ and WS₂–WSe₂ heterostructures through a lateral epitaxial process. Our studies indicate that simple sequential growth often fails to produce the desired heterostructures because the edge growth front can be easily passivated after termination of the first growth and exposure to ambient conditions. To retain a fresh, unpassivated edge growth front is important for successive lateral epitaxial growth. To this end, we have designed a thermal CVD process that allows in situ switching of the vapour-phase reactants to enable lateral epitaxial growth of single- or few-layer TMD lateral heterostructures. We used this technique to realize the growth of compositionally modulated MoS₂–MoSe₂ and WS₂–WSe₂ lateral heterostructures. The WS₂–WSe₂ lateral heterostuctures with both p- and n-type characteristics can also allow us to construct many other functional devices, for example, a CMOS inverter. The voltage gain of CMOS inverter reaches as large as 24.
To precisely control the band gap of these two-dimensional layered semiconductors is of central importance for creating optoelectronic devices with tunable spectral responses. Considering similarities in the atomic structure of many two dimensional semiconductor, such as MoS₂, MoSe₂, WS₂, WSe₂ , it is possible to create a mixed alloy system with a tunable band gap by alloy composition. A simple one-step chemical vapor deposition approach was developed for the simultaneous growth of alloy MoS_2xSe_2(1−x） triangular nanosheets with completely tunable composition and band gap. Also, lateral composition graded atomic layered 2D MoS_2(1−x)Se_2x nanosheets have been successfully synthesized using a simple moving source thermal evaporation method by an improved CVD route. Both microstructure and spectral characterizations demonstrate that the achieved nanosheets are highly crystallized, with the composition being continuously tuned from the pure MoS₂ at the center (x = 0) to a highly Se doped ternary alloy at the edge (x = 0.68).
References
1.Xidong Duan, Anlian Pan,Ruqin Yu, Xiangfeng Duan,et,a..l Nature Nanotechnology 9,2014,1024-1030.
2.Honglai Li, Xidong Duan,Anlian Pan, Xiangfeng Duan et,al.. J. Am. Chem. Soc. 2014, 136, 3756−3759
3.Honglai Li, Qinglin Zhang, Xidong Duan, Anlian Pan, Xiangfeng Duan et,al.. J. Am. Chem. Soc. 2015, 137, 5284−5287.

Molybdenum disulfide (MoS₂) transition metal dichalcogenide (TMDC) semiconductors have attracted increasing attention for electronic and optoelectronic applications because of their high electron mobility, high photosensitive, and tunable bandgap. Recently, a 0-D/2-D hybrid device was reported to have a high responsivity R (≤10⁷ A/W) and detectivity D^* (≤10¹⁵ Jones). However, a critical issue of the 0-D/2-D hybrid device is its long decay time (≥300 ms) induced from trap and leakage of residual carriers at the interfaces of hybrid devices. This issue remains a major challenge for the development of hybrid devices.

In this presentation, we first report a superior functional hybrid phototransistor fabricated using bilayer MoS₂ and colloidal CdSe NCs. The hybrid CdSe/MoS₂ phototransistor was designed to enhance the photodetector performance of the pristine MoS₂. The hybrid CdSe/MoS₂ phototransistor exhibited enhanced photoresponsivity R, 2.5 × 10⁵ A/W, photo-detectivity D^*, 2 × 10¹⁴ Jones, and decay time τ_decay, ~60 ms, compared with those of pristine MoS₂. The fast response of the photocurrent rise (τ_rise) and decay (τ_decay) times reported in our hybrid photodetector are orders of magnitude shorter than the values reported for other hybrid devices. To provide a detailed photodetection mechanism for this hybrid MoS₂ phototransistor, we discussed these effects via energy band diagrams of n-n type heterojunctions at the interface between bilayer MoS₂ and CdSe NCs by applying density functional theory (DFT) and examining the photoluminescence (PL) and absorption spectra of both materials.

Graphene quantum dots (GQDs) are considered as a promising green nanomaterial and an alternative to traditional fluorescent nanocrystals for applications in photocatalysts, sensors, and bioimaging. Therefore, integrating the excellent optical and electrical properties of GQDs and superior physical properties of graphene by the formation of hybrid structures should help generate unique physical properties and offer a potential pathway for constructing all graphene-based material devices with high performance. Here, we developed a novel ultraviolet (UV) sensor based on GQDs/reduced graphene oxide (RGO) hybrids, showing the high performances of photoresponsivity and detectivity. The GQDs prepared by the simple bottom-up approach show high quantum yield and can produce the photogenerated carriers due to the strong absorption of UV light.
GQDs were first synthesized by the carbonization of citric acid (CA) through the hydrothermal method with an average diameter of 4.4 nm and high crystallinity, suggesting that CA has been successfully transformed into high quality GQDs after hydrothermal treatment. For the UV sensor fabrication using GQDs/RGO hybrid, RGO film was prepared on a SiO₂/Si substrate using the spin-coating of GO and subsequent thermal reduction, and the GQDs solution was then sprayed onto the prepared RGO film. Under UV illumination, while the RGO film show no detectable current change, the GQDs/RGO hybrids clearly exhibits a current increase with the linear and symmetric I-V curves due to the Ohmic contact between GQDs and RGO. We further investigated the time-resolved photoresponse of the GQDs/RGO hybrids and the GQDs/RGO hybrids demonstrate good photoresponse and repeatability. As the UV light is on, the current increases sharply and reaches the saturation value. When the light source is off, the current recovers to its initial value. Notably, the UV sensor device using GQDs/RGO hybrids shows a high photo responsibility of 8.7×10² A/W and an excellent specific detectivity of 7.7×10¹³ Jones at a low operating voltage.
These high performances of the hybrids UV sensor are attributed to the strong absorption of UV light in GQDs, an efficient charge transfer at the GQDs/RGO junction, and a high electron mobility of RGO. The electrons of GQDs are excited to the conduction band under UV light illumination, and the photogenerated electrons transfer to the RGO film at the GQDs/RGO junction and flow through RGO to the Au electrode. Therefore, the recombination of photogenerated electron-hole pairs is reduced and the photocurrent of devices is enhanced. Our novel approach based on the solution-processed UV sensor using all carbon materials suggests an alternative strategy for designing the promising optoelectronic applications.

In this work, we developed a novel triboelectricity-assisted polymer-free method for the transfer of large-area chemical vapor deposited graphene films. With the assistance of the electrostatic force from friction generated charges, graphene sheet was successfully transferred from copper foils to flexible substrates. Characterization results confirm the presence of high quality graphene with less defects and contaminations, as compared to the graphene transferred by the conventional PMMA-mediated transfer process. What’s more, the graphene samples possess outstanding electrical transport capability and mechanical stability, which is manifested by acting as an electron transfer matrix in a flexible graphene/ZnO hybrid flexible photodetector. Our results show broad application potential of this transfer method in future flexible electronics and optoelectronics.

Semiconducting p-n junctions are the key building blocks for the electronic and optoelectronic devices. Carrier transport across the junction occurs by diffusion and drift processes and results in a depletion region, with a built-in potential characteristic of the semiconductor material properties and doping. It is possible to develop two dimensional (2D) ultra-thin p-n heterojunction by using van der Walls materials, which have received significant attention since the mechanical exfoliation graphene from parent graphite. Such van der Waals p-n heterojunction composed of p- and n-type semiconductors are predicted to exhibit completely different charge transport properties compared to their bulk heterojunction counterparts. So far the size of such p-n heterojunction and its practical implementations were limited by the size of the 2D materials to few tens of microns. Here, we show atomically thin 2D van der Waals p-n heterojunction synthesized by chemical vapor deposition technique over millimeter length scales. Atomically thin p–n heterojunctions were fabricated using van der Waals assembly of transition-metal dichalcogenides (TMDs) such as p-type WSe₂ and n-type MoS₂. We believe these atomically thin millimeter scale p−n heterojunction devices highlight the attractive prospects and opportunities of these materials to impact numerous optoelectronic device platforms, such as photodetectors, ultra-fast rectifiers, photoluminescence devices, electroluminescence devices, photovoltaics, as well as spin- and valley-polarized light emitting diodes and on-chip lasers.

Precise patterning of graphene is highly important for tailor-made and sophisticated two-dimensional nanoelectronic and optical devices. However, graphene-based heterostructures have been synthesized by delicate multi-step chemical vapor deposition methods, limiting preparation of versatile heterostructures. Here, we report single-step growth of graphene/amorphous carbon (a-C) heterostructures from a solid source of polystyrene through selective photo-crosslinking process. Graphene is successfully grown from neat polystyrene regions, while crosslinked polystyrene regions turn into a-C because of a large difference in their thermal stability. Since the electrical resistance of a-C is at least two orders of magnitude higher than that for graphene, the charge transport in graphene/a-C heterostructure occurs through the graphene area. Measurement of the quantum Hall effect in graphene/a-C lateral heterostructures clearly confirms the reliable quality of graphene and well-defined graphene/a-C interface. The ability to directly synthesize patterned graphene from polymer pattern opens up new possibilities for the preparation of versatile heterostructures.

The remarkable properties of graphene, a two dimensional sheet of carbon atoms, have brought tremendous attention and interest to other 2D materials including atomically thin 2D transition metal dichalcogenides (TMDCs), Black Phosphorous (BP) and hexagonal Boron Nitrides (hBN). These two dimensional materials exhibit different electronic properties, ranging from semi-metallic graphene and semiconducting transition metal dichalcogenides and black phosphorous to insulating boron nitride, due to the diversity in their energy bandgaps. The recent discovery of the narrow-bandgap black phosphorous ^[1] serendipitously bridges the energy gap between zero-bandgap graphene and the relatively large-bandgap TMDCs,making 2D material a promising candidate for optical application across a wide range of the electromagnetic spectrum including ultraviolet light (hBN, ~6eV), visible light (TMDCs, 1 ~ 2.5eV), near infrared light (BP, 0.3~2eV)and mid infrared light (graphene, 0eV) . A triple-layer heterostructure device is hence designed and fabricated by us, where BP is utilized as an active material for infrared detection and WSe₂ and MoS₂ are selected respectively for p-type and n-type channels. Besides the excellent quantum efficiency provided by the built-in electric field, the photo current is further amplified by the BJT-like npn-structure, inducing remarkable photo-responsivity and normalized photocurrent to dark current ratio.

Reference:
[1] Li, Likai, et al. "Black phosphorus field-effect transistors." Nature nanotechnology 9.5 (2014): 372-377.
[2] Xia, Fengnian, et al. "Two-dimensional material nanophotonics." Nature Photonics 8.12 (2014): 899-907.
[3] Zhang, Wenjing, et al. "Ultrahigh-gain photodetectors based on atomically thin graphene-MoS2 heterostructures." Scientific reports 4 (2014).
[4] Huang, Chunming, et al. "Lateral heterojunctions within monolayer MoSe2–WSe2 semiconductors." Nature materials 13.12 (2014): 1096-1101.

The synthetic route to produce SnS nanoplates with the Pbnm crystal structure without any surfactant. The process is very quick, environment-friendly and accomplished in a mild aqueous condition at low temperature. The synthesis involves two steps, formation of an intermediate tin oxide hydroxide (Sn₆O4(OH)₄) gel used as precursor and its chemical transformation into SnS nanoplates in the presence of sulfide precursor. we found that addition of small amount of PVP during the chemical transformation results in the formation of cubic SnS with the Cmcm crystal structure. We found self etching effect of SnS nanoplates as increasing reaction time. We discuss about the development of the crystal structures on the basis of density functional theory (DFT) calculations on the structure-energy relationship of SnS nanostructures. The optical properties of the SnS nanoplates and nanocubes are investigated respectively.

Aluminum gallium nitride (AlGaN) has attracted significant attention for deep ultraviolet (DUV) light emitting diodes (LEDs) and laser diodes (LDs), and is positioned to replace conventional mercury-based light sources for air purification, sterilizers, and bio-chemical detection. To date, however, the growth of high quality AlGaN nanowire has proved to be much more difficult, compared with gallium nitride (GaN) nanowire, because of its intrinsic properties. In this letter, we demonstrate that these critical challenges can be addressed by employing two-step metal-organic chemical vapor deposition (MOCVD) growth process, the AlGaN nanowires are directly grown on the low-temperature AlGaN seed that formed on low-cost Si substrate. The process temperature was varied between 710 and up to 1000 °C, which is a principal advantage in order to optimize the material properties of the high quality AlGaN nanowires. The doping process becomes significantly more complex with increasing the Al content and the electrical resistivity is high for n-type AlGaN doped by silicon and p-type AlGaN doped by magnesium. The materials characterization involved extensive implementation of field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), photoluminescence (PL), cathodoluminescence (CL) and x-ray diffraction (XRD) measurements. By carefully running synthesis conditions a record IQE of 80% can be realized with AlGaN nanowires, which is nearly equivalent compared to high quality GaN nanowires. The 365 nm emitting AlGaN nanowire LEDs (UV-A) were achieved, with a turn on voltage of about 8 V, which is significantly lower than the commonly observed 20-40 V. In the end, high performance AlGaN nanowire LEDs with emission wavelengths covering the UV-B/C bands will be also demonstrated.

Graphene,^[1] is viewed as a potential passive constituent of future optoelectronic devices^[2] due to its high electron mobility and optical transmittance (~97.7%) over a broad wavelength range but active graphene-based photonic devices have found limited applications due to its relatively inefficient interaction with light. Therefore an enhanced light matter interaction is required for their use in active photonic devices. We report a rapid, efficient and simple non-hazardous approach to synthesize large area graphene-Ag⁰ hybrid plasmonic nanostructures that show enhanced light-matter interaction. The novel hybrid combines the synergistic effect of the composite where Ag⁰ nanoparticles are stabilized in graphene matrix. The formation of highly stable Ag NPs with an average size of 40 nm is observed within the graphene layers. The Raman intensity of G band of graphene-Ag⁰ hybrid exhibits an enhancement by a factor of 20 over the control sample. The position of the G-band is up-shifted from 1583.6 cm^-1 to 1586 cm^-1, and 2D-band downshifted from 2738 cm^-1 to 2715 cm^-1 for the G-Ag⁰ hybrid, as compared to the control graphene sample indicating Ag NPs induced n-type doping of graphene-Ag⁰ nanocomposites due to metal-graphene interaction via electron transfer. Using optical simulation, we have studied the interaction of light with embedded Ag nanoparticles resulting in a strong amplification of the electric field in the graphene metal interface. We have fabricated Au/G-Ag⁰/Au lateral photodetector exhibiting high responsivity and spectral selectivity in the visible wavelength. The study exploits the synergistic effect of the graphene-metal nanocomposite, which may lead to the potential application of graphene based photonic devices.

Reference:
A. K. Geim, K. S. Novoselov, Nature Mater. 2007, 6(3), 183.
F.Bonaccorso, Z. Sun, T. Hasan, A. C. Ferrari, Nature Photon. 2010, 4, 611–622.

Molybdenum disulfide (MoS₂), a layered structure material, has some interesting features, such as, fairly large bandgap (∼1.2 - 2.2 eV), moderate mobility at ambient temperature (∼100 cm²V⁻¹s⁻¹), make it very promising candidate for optoelectronic applications. In this report, size tunable, luminescent MoS₂ quantum dots (QDs) have been successfully synthesized through well-controlled, yet inexpensive solvent assisted sono-chemical exfoliation method, followed by gradient centrifugation where the diameter of QDs could be varied within the range of ∼2 to 25 nm. Size tunable absorption and photoluminescence (PL) properties of MoS₂ below a critical dimension have been systematically observed and exhibited a red shift in energy with the increase of dot size. All the QD dispersions exhibited a strong absorption peak due to quantum size effect, along with two relatively weaker peaks at lower energy due to interband transition. The PL spectra have also shown an intense size dependent emission with the peak centered at 2.2-2.4 eV, originated from quantum confinement. On the other hand, relatively weaker peaks at ∼1.97 eV and ∼ 1.82 eV are attributed to the direct band-edge transitions. The energy difference (150-180 meV) between two low energy peaks is correlated to the valence band splitting, in excellent agreement with theoretically reported results.
We have also fabricated inorganic p-n heterojunction photodetector (PD) as well as light emitting diode (LED) using colloidal n-type MoS₂ QDs integrated with p-type Si. The PDs have shown very strong responsivity determined by the size of the QDs with optical selectivity in the visible to NIR wavelength range, which can be operated at a low applied bias at room temperature. The fabricated photodetector exhibited a high photo-to-dark current ratio (I_ON/I_OFF) of ~10³, with the peak responsivity and detectivity have been estimated to be ~850 mA/W and ~10¹¹ cm.Hz^1/2/W, respectively, at an applied bias of -2 V, for 2 nm dia. QDs. The responsivity of the device exhibited a dependency on QDs size and it’s enhanced by 4.5 times as the QDs size decreased from 7 nm to 2 nm. The devices also exhibited excellent stability and reproducibility with moderate response speed. The prepared QDs based photodetector is found to be attractive due to their unique properties arising from the geometry. Furthermore, the n-MoS₂/p-Si heterojunction also displayed electroluminescence (EL) characteristic with white light emissions spectra from ~400 nm to ~850 nm and the EL intensity increased with increasing forward bias. A bluish-white light emission could be directly observed by the naked eye, when sufficient forward bias is applied across the device. The EL emission could be detectable around 15 V and increases continuously at room temperature. Our work demonstrates that the great potential of a lithography-free, efficient approach for the fabrication PDs and LEDs on existing Si technology using colloidal MoS₂ QDs.

Two-dimensional heterostructures represent a group of emerging functional nanostructures that could be useful in future-generation electronics, optoelectronics, and energy harvesting. Here we demonstrated the growth of layered In2Se3 nanosheet on the top of graphene film using chemical-vapor-deposition (CVD) approach. The physical morphology of In2Se3 nanosheet can be adjusted through critical processing parameters in CVD deposition and pre-growth treatments. Extensive material characterization, including SEM, XPS, Raman, and XRD, are conducted to confirm the structure and composition of the synthesized van der Waals 2D heterostructure.

Graphene, a single layer of carbon atoms, has recently emerged as a novel material with unique electrical and optical properties for novel opto-electronic applications such as terahertz detection field effect transistors [1], ultra-broadband photodetectors [2], high efficient light emitting diodes [3] and barristors [4]. Most of these devices use of a Schottky barrier formed at a heterojunction between a semiconductor and the metallic graphene. Therefore, it is essential to understand the nature of carrier transport through Schottky barriers to design and optimization of graphene based novel devices.
Recently germanium (Ge) has received the much attention material for near infrared photo-detection due to its distinct properties including large absorption coefficient at near infrared frequencies, high electron conductivity compared to silicon (Si), low cost, and excellent compatibility of parallel processing with Si technology [5,6]. In addition, Ge growth on Si (Ge-on-Si) is more compatible with complementary metal-oxide-semiconductor (CMOS) circuitry and Si based monolithic integration on a same chip because of its relatively straightforward fabrication technique when compared with free-standing bulk Ge substrate [7]. In present work, we fabricated infrared photodetectors with graphene-germanium-graphene structures on epitaxial grown Ge-on-Si substrate and investigated its opto-electrical parameters. The photodetectors operation explained by using band-alignment diagram and attributed to a charge transport mechanism between graphene and epitaxial Ge film on Si. The fabricated device shows dark current of 2.01 µA, photo responsivity R of about 0.35 A/W, detectivity D of 3.86×10¹⁰ Jones (cmHz^1/2/W), and a quantum efficiency η of 28 % at -2 V reverse bias, which demonstrates the high value of the monolayer graphene electrode towards infrared detection. The integration of graphene electrodes into conventional Si based technology compatible Ge is therefore promising candidates for potential applications in optoelectronic circuits and low cost, high performance, and reliable photodetectors.