Controllable deposition of organic metal halide perovskite films with wafer-scale uniformity by single source flash evaporation

Conventional solution-processing techniques such as the spin-coating method have been used successfully to reveal excellent properties of organic-inorganic halide perovskites (OHPs) for optoelectronic devices such as solar cell and light-emitting diode, but it is essential to explore other deposition techniques compatible with large-scale production. Single-source flash evaporation technique, in which a single source of materials of interest is rapidly heated to be deposited in a few seconds, is one of the candidate techniques for large-scale thin film deposition of OHPs. In this work, we investigated the reliability and controllability of the single-source flash evaporation technique for methylammonium lead iodide (MAPbI(3)) perovskite. In-depth statistical analysis was employed to demonstrate that the MAPbI(3) films prepared via the flash evaporation have an ultrasmooth surface and uniform thickness throughout the 4-inch wafer scale. We also show that the thickness and grain size of the MAPbI(3) film can be controlled by adjusting the amount of the source and number of deposition steps. Finally, the excellent large-area uniformity of the physical properties of the deposited thin films can be transferred to the uniformity in the device performance of MAPbI(3) photodetectors prepared by flash evaporation which exhibited the responsivity of 0.2 A/W and detectivity of 3.82x10(11) Jones.

Effect of Facile p-Doping on Electrical and Optoelectronic Characteristics of Ambipolar WSe2 Field-Effect Transistors

We investigated the electrical and optoelectronic characteristics of ambipolar WSe2 field-effect transistors (FETs) via facile p-doping process during the thermal annealing in ambient. Through this annealing, the oxygen molecules were successfully doped into the WSe2 surface, which ensured higher p-type conductivity and the shift of the transfer curve to the positive gate voltage direction. Besides, considerably improved photoswitching response characteristics of ambipolar WSe2 FETs were achieved by the annealing in ambient. To explore the origin of the changes in electrical and optoelectronic properties, the analyses via X-ray photoelectron, Raman, and photoluminescence spectroscopies were performed. From these analyses, it turned out that WO3 layers formed by the annealing in ambient introduced p-doping to ambipolar WSe2 FETs, and disorders originated from the WO3/WSe2 interfaces acted as non-radiative recombination sites, leading to significantly improved photoswitching response time characteristics.The authors appreciate the financial support of the National Creative Research Laboratory program (Grant No. 2012026372) through the National Research Foundation of Korea funded by the Korean Ministry of Science and ICT

Electronic properties of gold nanoclusters/semiconductor structures with low resistance interfaces

Self-assembled metal/molecule/semiconductor nanostructures are utilized to define nanoelectronic device contact structures and are characterized using ultra high vacuum (UHV) scanning tunneling microscopy (STM). As examples of the controlled nanostructures for nanoelectronic device applications, nonalloyed ohmic contact nanostructures have been utilized on a surface layer of LTG:GaAs, i.e., GaAs grown at a low temperature by molecular beam epitaxy. The controlled-geometry nanocontact is obtained by depositing a 4 nm diameter single crystal Au cluster (truncated octahedral shape) onto n-GaAs(100) having LTG:GaAs based ohmic contact layers using ex-situ chemical self-assembly techniques. A self-assembled monolayer (SAM) of xylyl dithiol (HS-CH2-C6H4-CH2-SH) is formed on LTG:GaAs and provides an effective organic metal/semiconductor interface having both a robust mechanical tethering and a strong electronic coupling between the Au nanoclusters and the LTG:GaAs surface. UHV STM is used to locate and probe the electronic properties of the nanocontacts. STM current versus voltage (I-V) data measured over Au nanoclusters exhibit an ohmic behavior with a significant enhancement in the conduction for low bias voltages compared to I-V data over the SAM-coated LTG:GaAs substrate. A specific contact resistance of 10 −6–10−7 Ω·cm 2 and a current density of 106–107 A/cm2 have been measured on the nanocontacts from STM. The ohmic nanocontact is mainly due to the sequential tunneling through the xylyl dithiol layer and the LTG:GaAs layer while a midgap band of defect states in the LTG:GaAs layer assists conduction as if it effectively reduces the barrier width. Another interesting approach is to combine the nanoscale elements (Au clusters) and ordering from self-assembly processes with a procedure which can impose an arbitrary larger-scale pattern to form the specific configurations and interconnections needed for computation. Toward this goal, high-quality hexagonal close-packed arrays of Au nanoclusters (5 nm in diameter) are formed within patterned regions on active GaAs substrates having LTG: GaAs cap layer and are characterized using STM. This approach utilizes a patterned template which guides Au nanoclusters into pre-selected regions with xylyl dithiol. The directed self-assembly techniques used to fabricate these structures have the potential to provide high-throughput fabrication of nanostructures for nanoelectronics and other nanoscale applications

Layer-by-Layer Structural Identification of 2D Ruddlesden-Popper Hybrid Lead Iodide Perovskites by Solid-State NMR Spectroscopy

Application of two-dimensional (2D) organic-inorganic hybrid halide perovskites for optoelectronic devices requires detailed understanding of the local structural features including the Pb-I bonding in the 2D layers and the capping ligand-perovskite interaction. In this study, we show that 1H and 207Pb solid-state Nuclear Magnetic Resonance (NMR) spectroscopy can serve as a non-invasive and complementary technique to quantify the composition and to probe the local structural features of 2D Ruddlesden-Popper phase BA2MAn-1PbnI3n+1 (n=1-4) with butylammonium (BA) spacers. 207Pb echo and 1H-detected 207Pb→1H heteronuclear correlation (HETCOR) experiments enables layer-by-layer structural detection of 2D halide perovskites. We show that the observed correlation between 207Pb NMR shifts and mean Pb-I bond lengths around each Pb site allows us to probe the local bonding environment of Pb via its 207Pb NMR shift. We envisage that this technique will be vital for better understanding the materials properties as determined by the local atomistic environments in multi-dimensional halide perovskites