Synthesis of One Atom Thick, Two-Dimensional Gold Crystals and Their Novel Properties

We report, for the first time, a technique to synthesize free standing, one atom thick two-dimensional gold crystals (i.e. <i>goldene</i>) and self-assembled one atom thick, 1-d and 2-d superlattice arrays of gold. Atomic Force Microscopy confirmed the thickness of these crystals and arrays to be ~ 1 - 2Å. High Resolution Transmission Electron Microscopy (HREM) lattice imaging data and Selective Area Electron Diffraction (SAED) showing the forbidden reflection from the fractional plane 1/3{422} confirmed that these are one atom thick crystals. Energy Dispersive X-Ray Spectroscopy (EDS) unequivocally confirmed the chemical identity of these structures to be gold. Some of these crystals self-assembled to form nano-ring structures and Magnetic Force Microscopy (MFM) measurements indicated that they are magnetic. Current/voltage measurements of the nano-rings using a micro two-point probe system indicated that they are insulators. Momentary application of a point force using an atomic force microscope on <b><i>a</i></b> nano-ring resulted every nano-ring within the entire scanner area to create several replicas. Over time, <i>without</i> the influence of any applied external force or stimulus, each replica returned back to its <i>original structure</i> and re-created its original form. Under specific experimental conditions we observed an apparent spatial and temporal correlation between the nano-rings and the replicas. These observations indicate that the self-assembled 2D gold crystals in the nano-rings are held together by a weak magnetic force which is consistent with the MFM observation.<br/><br/>Current-Voltage (IV) spectroscopy using conductive AFM (C-AFM) revealed that 2D gold on silicon substrates is also an insulator. Application of a localized e-field (i.e. C-AFM tip bias voltage) at room temperature, resulted in the alignment of the 2D crystals into stable patterns implying the presence of axial charge polarization. The crystal morphology of 2D gold reversibly changed from thin films to ellipsoids to hexagons as a function of the applied tip bias voltage.