Saifeng Zhang

Liquid exfoliation of solvent-stabilized few-layer black phosphorus for applications beyond electronics.

Few-layer black phosphorus (BP) is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of environmental stability severely limits its synthesis and processing. Here we demonstrate that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions in solvents such as N-cyclohexyl-2-pyrrolidone (CHP). Nanosheets are surprisingly stable in CHP, probably due to the solvation shell protecting the nanosheets from reacting with water or oxygen. Experiments, supported by simulations, show reactions to occur only at the nanosheet edge, with the rate and extent of the reaction dependent on the water/oxygen content. We demonstrate that liquid-exfoliated BP nanosheets are potentially useful in a range of applications from ultrafast saturable absorbers to gas sensors to fillers for composite reinforcement.

Direct Observation of Degenerate Two-Photon Absorption and Its Saturation in WS2 and MoS2 Monolayer and Few-Layer Films

The optical nonlinearity of WS2 and MoS2 monolayer and few-layer films was investigated using the Z-scan technique with femtosecond pulses from the visible to the near-infrared range. The nonlinear absorption of few- and multilayer WS2 and MoS2 films and their dependences on excitation wavelength were studied. WS2 films with 1-3 layers exhibited a giant two-photon absorption (TPA) coefficient as high as (1.0 ± 0.8) × 10(4) cm/GW. TPA saturation was observed for the WS2 film with 1-3 layers and for the MoS2 film with 25-27 layers. The giant nonlinearity of WS2 and MoS2 films is attributed to a two-dimensional confinement, a giant exciton effect, and the band edge resonance of TPA.

Buffer-layer-induced barrier reduction: Role of tunneling in organic light-emitting devices

Based on the WKB approximation of the tunneling model, we calculate the J–V characteristics of organic light-emitting devices (OLEDs) having buffer layers of different thickness. The results show how the insertion of a buffer layer with proper thickness lowers the OLED turn-on voltage. Further calculation suggests some parameters, such as the resistivity ratio and the position of the conduction band minimum of the buffer layer relative to the lowest unoccupied molecular orbital of the organic layer, are important in selecting a buffer material. A quantitative estimation of the optimal buffer layer thickness is also presented to serve as a guide to device design. The model is validated by comparison of its predictions to experimental results.

Enhancement of electron injection in organic light-emitting devices using an Ag/LiF cathode

A LiF-buffered silver cathode has been used in organic light-emitting devices (OLEDs) with structure indium–tin–oxide/N,N′-bis-(1-naphthl)-diphenyl-1,1′-biphenyl-4,4′-diamine (50 nm)/Alq3 (100 nm)/cathode. The efficiency of electron injection from the cathode is strongly dependent on the thickness of the LiF buffer layer. While a LiF layer thinner than 1.0 nm leads to higher turn-on voltage and decreased electroluminescent (EL) efficiency, a LiF layer of 3.0 nm significantly enhances the electron injection and results in lower turn-on voltage and increased EL efficiency. A brightness of 16 000 cd/m2 and EL efficiency of 4.8 cd/A can be achieved with an Ag/LiF cathode. This dependence of electron injection on the LiF thickness is quite different from that reported for OLEDs with a Al/LiF cathode, but can be well understood using the tunneling model.

Electron blocking and hole injection: The role of N, N'-bis(naphthalen-1-y)-N, N'-bis(phenyl)benzidine in organic light-emitting devices

The current density-luminance-voltage characteristics of organic light-emitting devices (OLEDs) with N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine (NPB) of various thicknesses as the hole transport layer have been investigated. It is found that for conventional structures of indium–tin–oxide/NPB/tris(8-hydroxyquinoline) aluminum (Alq3) (60 nm)/LiF (0.5 nm)/Al the optimal hole injection and luminescence efficiencies appear at NPB thicknesses of 5 and 20 nm, respectively. The large difference between the two optimal thicknesses suggests that the effective block of the NPB layer against electrons from across the Alq3/NPB interface is essential for high-efficiency operation of the OLEDs. The electron blocking effect of NPB is further confirmed by the electroluminescence (EL) behavior of devices with the structure of ITO/NPB(5 nm)/Alq3:4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) (30 nm)/NPB/Alq3(60 nm)/LiF(0.5 nm)/Al. The proportion of DCM EL to the whole EL decreases with inc...

Optical Limiting and Theoretical Modelling of Layered Transition Metal Dichalcogenide Nanosheets.

Nonlinear optical property of transition metal dichalcogenide (TMDC) nanosheet dispersions, including MoS2, MoSe2, WS2, and WSe2, was performed by using Z-scan technique with ns pulsed laser at 1064 nm and 532 nm. The results demonstrate that the TMDC dispersions exhibit significant optical limiting response at 1064 nm due to nonlinear scattering, in contrast to the combined effect of both saturable absorption and nonlinear scattering at 532 nm. Selenium compounds show better optical limiting performance than that of the sulfides in the near infrared. A liquid dispersion system based theoretical modelling is proposed to estimate the number density of the nanosheet dispersions, the relationship between incident laser fluence and the size of the laser generated micro-bubbles, and hence the Mie scattering-induced broadband optical limiting behavior in the TMDC dispersions.

Role of hole playing in improving performance of organic light-emitting devices with an Al2O3 layer inserted at the cathode-organic interface

The role of hole playing in improving electron injection in the presence of an Al2O3 layer at the organic-cathode interface is discussed. It is deduced that, according to the model of tunneling barrier reduction and the carrier transporting mechanism in organic light-emitting devices, electron injection will be enhanced only if holes are injected and accumulated at the organic-buffer interface. The validity of this analysis is well confirmed by experimental results. The observed abnormal characteristic of operating voltage varying with the Al2O3 layer thickness and the efficiency improvement are also well explained by the model.

In situ photoluminescence investigation of doped Alq

We report the photoluminescence (PL) properties measured in situ from vacuum-deposited organic films of tris-(8-hydroxyquinoline) aluminum (Alq) doped with 4-(dicyanomethylene)-2-methyl-6(p-dimethylaminostyryl)-4H-pyran (DCM), where the red emission from the guest molecules is due to Forster energy transfer of excited state energy from host to guest. Both bare DCM-doped Alq (Alq:DCM) and bilayer Alq/Alq:DCM films have been studied, with the thickness of the Alq overlayer continuously varied in the latter case. The PL spectra from the bilayer structure contain no Alq contribution when its thickness is below 2.4 nm. Taking the value as the maximum distance for which the Alq exciton can travel in the film and still transfer its energy to a DCM molecule, the minimum DCM concentration in Alq:DCM necessary to produce red emission only can be estimated at 0.31 wt %. The most efficient red emission appears at the DCM concentration of about 1.7 wt %, at which more than 90% Alq-originated excitons are involved in t...

Dual role of LiF as a hole-injection buffer in organic light-emitting diodes

It is demonstrated experimentally that the effect of a LiF buffer layer inserted at the ITO\N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′ biphenyl 4,4′-dimaine (NPB) interface on the hole injection is greatly dependent on the initial barrier height (IBH) existing at the interface. Only for a large IBH, will the introduction of the LiF show improvement effect. For small one, it will weaken the hole injection. These phenomena are explained in terms of tunneling model and calculations based on this model show a good agreement with the experimental results. This further confirms that the energy level realignment and the change in carrier tunneling probability are mainly responsible for the variation of current injection induced by the insulating buffers in organic light-emitting diodes.

Sodium stearate, an effective amphiphilic molecule buffer material between organic and metal layers in organic light-emitting devices

Tris (8-hydroxyquinoline) aluminum (Alq3)-based organic light-emitting devices using an amphiphilic molecule sodium stearate (NaSt) layer between aluminum (Al) cathode and Alq3 have been fabricated. By comparing the devices with those containing a LiF buffer layer, the results demonstrate that both have almost the same high electroluminescent (EL) brightness but the former is more stable. The amphiphilic property of NaSt is considered as the main reason for this enhancement.