Perovskite Quantum Dots Lasing in Double-Heterostructure through Energy Transfer.

Planar double heterostructures were initially investigated and have been successfully applied in III-V semiconductor lasers due to their excellent roles in confining both the photons and carriers. Here, we design and fabricate a (PEA)2Csn-1PbnX3n+1 (quasi-2D)/CsPbBr3 QD/quasi-2D double-heterostructure sandwiched in a 3/2 λ DBR microcavity, and then demonstrate a single-mode pure-green lasing with a threshold of 53.7 µJ/cm2 under nanosecond-pulsed optical pumping. The thresholds of these heterostructure devices decrease statistically by about 50% compared to the control group with no energy donor layers, PMMA/QD/PMMA in an identical microcavity. We show that there is efficient energy transfer from the barrier regions of the quasi-2D phases to the QD layer by transient absorption and luminescence lifetime spectra and that such energy transfer leads to marked threshold reduction. This work indicates that the double-heterostructure configurations should play a significant role in the future perovskite electrically pumped laser.

Nanoscale Channel Length MoS2 Vertical Field-Effect Transistor Arrays with Side-Wall Source/Drain Electrodes.

Two-dimensional transition metal dichalcogenides (TMDCs) have natural advantages in overcoming the short-channel effect in field-effect transistors (FETs) and in fabricating three-dimensional FETs, which benefit in increasing device density. However, so far, most reported works related to MoS2 FETs with a sub-100 nm channel employ mechanically exfoliated materials and all of the works involve electron beam lithography (EBL), which may limit their application in fabricating wafer-scale device arrays as demanded in integrated circuits (ICs). In this work, MoS2 FET arrays with a side-wall source and drain electrodes vertically distributed are designed and fabricated. The channel length of the as-fabricated FET is basically determined by the thickness of an insulating layer between the source and drain electrodes. The vertically distributed source and drain electrodes enable to reduce the electrode-occupied area and increase in the device density. The as-fabricated vertical FETs exhibit on/off ratios comparable to those of mechanically exfoliated MoS2 FETs with a nanoscale channel length under identical VDS. In addition, the as-fabricated FETs can work at a VDS as low as 10 mV with a desirable on/off ratio (1.9 × 107), which benefits in developing low-power devices. Moreover, the fabrication process is free from EBL and can be applied to wafer-scale device arrays. The statistical results show that the fabricated FET arrays have a device yield of 87.5% and an average on/off ratio of about 1.7 × 106 at a VDS of 10 mV, with the lowest and highest ones to be about 1.3 × 104 and 1.9 × 107, respectively, demonstrating the good reliability of our fabrication process. Our work promises a bright future for TMDCs in realizing high-density and low-power nanoelectronic devices in ICs.

High-Performance CMOS Inverter Array with Monolithic 3D Architecture Based on CVD-Grown n-MoS2 and p-MoTe2.

In this work, monolithic three-dimensional complementary metal oxide semiconductor (CMOS) inverter array has been fabricated, based on large-scale n-MoS2 and p-MoTe2 grown by the chemical vapor deposition method. In the CMOS device, the n- and p-channel field-effect transistors (FETs) stack vertically and share the same gate electrode. High k HfO2 is used as the gate dielectric. An Al2 O3 seed layer is used to protect the MoS2 from heavily n-doping in the later-on atomic layer deposition process. P-MoTe2 FET is intentionally designed as the upper layer. Because p-doping of MoTe2 results from oxygen and water in the air, this design can guarantee a higher hole density of MoTe2 . An HfO2 capping layer is employed to further balance the transfer curves of n- and p-channel FETs and improve the performance of the inverter. The typical gain and power consumption of the CMOS devices are about 4.2 and 0.11 nW, respectively, at VDD of 1 V. The statistical results show that the CMOS array is with high device yield (60%) and an average voltage gain value of about 3.6 at VDD of 1 V. This work demonstrates the advantage of two-dimensional semi-conductive transition metal dichalcogenides in fabricating high-density integrated circuits.

Large-area large-grain CsPbCl3perovskite films by confined re-growth for violet photodetectors.

CsPbCl3perovskite is an attractive semiconductor material with characteristics such as a wide bandgap, high chemical stability, and excellent optoelectronic properties, which broaden its application prospects for ultraviolet (UV) and violet photodetectors (PDs). However, large-area CsPbCl3films with high coverage, large grains, and controllable thickness are still difficult to prepare by using the solution method due to the extremely low solubility of their precursors in conventional solvents. Herein, a water-assisted confined re-growth method is developed, and a CsPbCl3microcrystalline film with an area of 3 cm × 3 cm is grown, the thickness of which is controllable within a range of several microns. The as-prepared thin film exhibits a flat and smooth surface, large grains, and enhanced photoluminescence. Furthermore, the fabricated violet PDs based on the prepared CsPbCl3film show a high responsivity of 2.17 A W-1, external quantum efficiency of 664%, on/off ratio of 2.58 × 103, and good stability. This study provides a prospective solution for the growth of large-area, large-grain, and surface-smooth CsPbCl3films for high-performance UV and violet PDs.

Seeded 2D epitaxy of large-area single-crystal films of the van der Waals semiconductor 2H MoTe2.

The integration of two-dimensional (2D) van der Waals semiconductors into silicon electronics technology will require the production of large-scale, uniform, and highly crystalline films. We report a route for synthesizing wafer-scale single-crystalline 2H molybdenum ditelluride (MoTe2) semiconductors on an amorphous insulating substrate. In-plane 2D-epitaxy growth by tellurizing was triggered from a deliberately implanted single seed crystal. The resulting single-crystalline film completely covered a 2.5-centimeter wafer with excellent uniformity. The 2H MoTe2 2D single-crystalline film can use itself as a template for further rapid epitaxy in a vertical manner. Transistor arrays fabricated with the as-prepared 2H MoTe2 single crystals exhibited high electrical performance, with excellent uniformity and 100% device yield.

Growth of hopper-shaped CsPbCl3 crystals and their exciton polariton emission.

CsPbCl3 is an attractive wide-bandgap perovskite semiconductor. Herein, we have grown hopper-shaped CsPbCl3 crystals in a solution droplet dripped on a heated substrate. During the growth, we have observed the impacts of the coffee ring effect and Marangoni flow, which may result in the hopper shape. Their photoluminescence spectra feature double peaks, which are located at 413.9 nm and 422.0 nm, respectively, and the latter increases faster in intensity than the former as the excitation power increases. We believe that the higher-energy peak originates from the excitonic emission and the lower-energy one is from the polaritons' emission, where the polaritons are generated in the exciton-exciton inelastic scattering process. Based on such an explanation, the exciton binding energy of CsPbCl3 is estimated to be 76.7 meV in our experiments, consistent with the previous reports.

Atomic-Precision Repair of a Few-Layer 2H-MoTe2 Thin Film by Phase Transition and Recrystallization Induced by a Heterophase Interface.

2D semiconductors have emerged as promising candidates for post-silicon nanoelectronics, owing to their unique properties and atomic thickness. However, in the handling of 2D material, various forms of macroscopic damage, such as cracks, wrinkles, and scratches, etc., are usually introduced, which cause adverse effects on the material properties and device performance. Repairing such macroscopic damage is crucial for improving device performance and reliability, especially for large-scale 2D device arrays. Here, a method is demonstrated repair damage to few-layer 2H-MoTe2 films with atomic precision, and its mechanism is elucidated. The repaired 2H-MoTe2 inherits the lattice orientation of the adjacent original 2H-MoTe2 , thereby forming an atomically perfect lattice at the repaired interface. The time-evolution experiments show that the interface between the 2H- and early formed 1T'-MoTe2 plays an important role in the subsequent phase transition and recrystallization. Electrical measurements on the original MoTe2 , repaired MoTe2 , and cross-interface regions show unobservable differences, indicating that the repaired MoTe2 has the same electrical quality as the original one and the interface does not introduce extra scattering centers for carrier transport. The findings provide an effective strategy for macroscopic damage repair of few-layer 2H-MoTe2 , which paves the way for its practical application in advanced electronics and optoelectronics.

Ultra-Shallow Doping of GaAs with Mg, Cr, Mn and B Using Plasma Stimulated Room-Temperature Diffusion.

It is demonstrated that Mg, Cr, Mn and B can be doped close to GaAs surface by plasma doping without external bias at room temperature (RT). The process only takes a few minutes, and impurity densities in the range of 1018-1021/cm3 can be achieved with doping depths about twenty nanometers. The experiment results are analyzed and the physical mechanism is tentatively explained as follows: during the doping process, impurity ion implantation under plasma sheath voltage takes place, simultaneously, plasma stimulates RT diffusion of impurity atom, which plays the main role in the doping process. The enhanced RT diffusion coefficients of Mg, Cr, Mn and B in GaAs are all in the order of magnitude of 10-15 cm2sec-1. This is reported for the first time among all kinds of plasma assisted doping methods.

Scaling-up Atomically Thin Coplanar Semiconductor-Metal Circuitry via Phase Engineered Chemical Assembly.

Two-dimensional (2D) layered semiconductors, with their ultimate atomic thickness, have shown promise to scale down transistors for modern integrated circuitry. However, the electrical contacts that connect these materials with external bulky metals are usually unsatisfactory, which limits the transistor performance. Recently, contacting 2D semiconductors using coplanar 2D conductors has shown promise in reducing the problematic high contact resistance. However, many of these methods are not ideal for scaled production. Here, we report on the large-scale, spatially controlled chemical assembly of the integrated 2H-MoTe2 field-effect transistors (FETs) with coplanar metallic 1T'-MoTe2 contacts via phase engineered approaches. We demonstrate that the heterophase FETs exhibit ohmic contact behavior with low contact resistance, resulting from the coplanar seamless contact between 2H and 1T'-MoTe2 confirmed by transmission electron microscopy characterizations. The average mobility of the heterophase FETs was measured to be as high as 23 cm2 V-1 s-1 (comparable with those of exfoliated single crystals), due to the large 2H-MoTe2 single-crystalline domain size (486 ± 187 µm). By developing a patterned growth method, we realize the 1T'-MoTe2 gated heterophase FET array whose components of the channel, gate, and contacts are all 2D materials. Finally, we transfer the heterophase device array onto a flexible substrate and demonstrate the near-infrared photoresponse with high photoresponsivity (∼1.02 A/W). Our study provides a basis for the large-scale application of phase-engineered coplanar MoTe2 semiconductor-metal structure in advanced electronics and optoelectronics.

Millimeter-Scale Single-Crystalline Semiconducting MoTe2 via Solid-to-Solid Phase Transformation.

Among the Mo- and W-based two-dimensional (2D) transition metal dichalcogenides, MoTe2 is particularly interesting for phase-engineering applications, because it has the smallest free energy difference between the semiconducting 2H phase and metallic 1T' phase. In this work, we reveal that, under the proper circumstance, Mo and Te atoms can rearrange themselves to transform from a polycrystalline 1T' phase into a single-crystalline 2H phase in a large scale. We manifest the mechanisms of the solid-to-solid transformation by conducting density functional theory calculations, transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. The phase transformation is well described by the time-temperature-transformation diagram. By optimizing the kinetic rates of nucleation and crystal growth, we have synthesized a single-crystalline 2H-MoTe2 domain with a diameter of 2.34 mm, a centimeter-scale 2H-MoTe2 thin film with a domain size up to several hundred micrometers, and a seamless 1T'-2H MoTe2 coplanar homojunction. The 1T'-2H MoTe2 homojunction provides an elegant solution for ohmic contact of 2D semiconductors. The controlled solid-to-solid phase transformation in 2D limit provides a new route to realize wafer-scale single-crystalline 2D semiconductor and coplanar heterostructure for 2D circuitry.

Direct synthesis of high-quality perovskite nanocrystals on a flexible substrate and deterministic transfer.

Solid-state perovskite nanocrystals are promising coherent light sources, as there is optical feedback within the crystal structure. In order to utilize the high performance of perovskites for on-chip applications, or observe new physical phenomena, these crystals must be integrated with pre-fabricated electronic or photonic structures. However, the material's fragility has made the deterministic transfer a great challenge thus far. Here, we report the first deterministic transfer of perovskite nanocrystals with sub-micron accuracy. Cesium lead halide (CsPbI3) nanocrystals were directly synthesized on flexible polydimethylsiloxane (PDMS) stamps via chemical vapor deposition (CVD) and subsequently transferred onto arbitrary substrates/structures. We demonstrated the transfer of a CsPbI3 crystalline nanoplate (NP) onto an 8 µm fiber core and achieved single-mode whispering gallery mode lasing. Our method can be extended to a variety of other arbitrary substrates (e.g., electrodes, photonic structures, micromechanical systems), laying the foundations for previously unattainable opportunities in perovskites-based devices.

Fully Transparent Quantum Dot Light-Emitting Diode with a Laminated Top Graphene Anode.

A new method to employ graphene as top electrode was introduced, and based on that, fully transparent quantum dot light-emitting diodes (T-QLEDs) were successfully fabricated through a lamination process. We adopted the widely used wet transfer method to transfer bilayer graphene (BG) on polydimethylsiloxane/polyethylene terephthalate (PDMS/PET) substrate. The sheet resistance of graphene reduced to ∼540 Ω/□ through transferring BG for 3 times on the PDMS/PET. The T-QLED has an inverted device structure of glass/indium tin oxide (ITO)/ZnO nanoparticles/(CdSSe/ZnS quantum dots (QDs))/1,1-bis[(di-4-tolylamino)phenyl] cyclohexane (TAPC)/MoO3/graphene/PDMS/PET. The graphene anode on PDMS/PET substrate can be directly laminated on the MoO3/TAPC/(CdSSe/ZnS QDs)/ZnO nanoparticles/ITO/glass, which relied on the van der Waals interaction between the graphene/PDMS and the MoO3. The transmittance of the T-QLED is 79.4% at its main electroluminescence peak wavelength of 622 nm.

Efficient small molecular organic light emitting diode with graphene cathode covered by a Sm layer with nano-hollows and n-doped by Bphen:Cs2CO3 in the hollows.

Graphene is a favorable candidate for electrodes of organic light emitting diodes (OLEDs). Graphene has quite a high work function of ∼4.5 eV, and has been extensively studied when used as anodes of OLEDs. In order to use graphene as a cathode, the electron injection barrier between the graphene cathode and the electron transport layer has to be low enough. Using 4,7-diphenyl-1,10-phenanthroline (Bphen):Cs2CO3 to n-dope graphene is a very good method, but the electron injection barrier between the n-doped graphene and Bphen:Cs2CO3 is still too high to be ∼1.0 eV. In this work, in order to further reduce the electron injection barrier, a novel method is suggested. On the graphene cathode, a Sm layer with a lot of nano-hollows, and subsequently a layer of Bphen:Cs2CO3, are deposited. The Bphen:Cs2CO3 can n-dope graphene in the nano-hollows, and the Fermi level of the graphene rises. The nano Sm layer is very easily oxidized. Oxygen adsorbed on the surface of graphene may react with Sm to form an O--Sm+ dipole layer. On the areas of the Sm oxide dipole layer without nano-hollows, the electron injection barrier can be further lowered by the dipole layer. Electrons tend to mainly inject through the lower electron barrier where the dipole layer exists. Based on this idea, an effective inverted small molecular OLED with the structure of graphene/1 nm Sm layer with a lot of nano-hollows/Bphen:Cs2CO3/Alq3:C545T/NPB/MoO3/Al is presented. The maximum current efficiency and maximum power efficiency of the OLED with a 1 nm Sm layer are about two and three times of those of the reference OLED without any Sm layer, respectively.

Self-powered flexible and transparent photovoltaic detectors based on CdSe nanobelt/graphene Schottky junctions.

Flexible and transparent electronic and optoelectronic devices have attracted more and more research interest due to their potential applications in developing portable, wearable, low-cost, and implantable devices. We have fabricated and studied high-performance flexible and transparent CdSe nanobelt (NB)/graphene Schottky junction self-powered photovoltaic detectors for the first time. Under 633 nm light illumination, typical photosensitivity and responsivity of the devices are about 1.2 × 10(5) and 8.7 A W(-1), respectively. Under 3500 Hz switching frequency, the response and recovery times of them are about 70 and 137 µs, respectively, which, to the best of our knowledge, are the best reported values for nanomaterial based Schottky junction photodetectors up to date. The detailed properties of the photodetectors, such as the influences of incident light wavelength and light intensity on the external quantum efficiency and speed, are also investigated. Detailed discussions are made in order to understand the observed phenomena. Our work demonstrates that the self-powered flexible and transparent CdSe NB/graphene Schottky junction photovoltaic detectors have a bright application prospect.

[Effects of Ag nanocrystals on electroluminescence in Si oxide films].

Ag nanocrystal-embedded silicon oxide (SiO2 : Ag) films with varying Ag fractions were prepared on p-Si substrate by magnetron co-sputtering and thermal annealing. Visible electroluminescence (EL) was observed from the structures of ITO/SiO2 : Ag/p-Si. The authors found that Ag nanocrystals in the SiO2 film can not only shift the EL peak evidently but also enhance the EL intensity markedly. The larger the Ag fractions in the EL structures, the longer the peak wavelengths. The electromagnetic interactions of the Ag nanocrystals with the emitters in the film via local surface plasmons are considered responsible for these experimental results.

Ultrahigh-performance inverters based on CdS nanobelts.

We report ultrahigh-performance inverters, each consisting of two top-gate metal-oxide-semiconductor field-effect transistors based on n-CdS nanobelts. High-kappa HfO(2) dielectrics are used as the top-gate oxide layers. The inverters have a large supply voltage (V(DD)) range (from 50 mV to 10 V) and very high voltage gain ( approximately 10, 100, and 1000 at V(DD) = 0.2, 1, and 10 V, respectively). Current consumption is less than 7 nA at V(DD) = 1 V, corresponding to a power consumption of less than 7 nW. The high and low output voltages are close to full rail. The inverters also exhibit good dynamic behavior with square wave input at frequencies up to 1 kHz. The operation of the inverters is analyzed in detail. The inverters are promising for future low power high performance logic circuit applications.

High-performance logic circuits constructed on single CdS nanowires.

A high-performance NOT logic gate (inverter) was constructed by combining two identical n-channel metal-semiconductor field-effect transistors (MESFETs) made on a single CdS nanowire (NW). The inverter has a voltage gain as high as 83, which is the highest reported so far for inverters made on one-dimensional nanomaterials. The MESFETs used in the inverter circuit show excellent transistor performance, such as high on/off current ratio ( approximately 10(7)), low threshold voltage ( approximately -0.4 V), and low subthreshold swing ( approximately 60 mV/dec). With the assembly of three identical NW MESFETs, NOR and NAND gates have been constructed.