CdS Nanosheets Research Articles

Two-dimensional CdS nanosheet-based TFT and LED nanodevices

Semiconductor nanosheets have several unique applications in electronic and optoelectronic nanodevices. We have successfully synthesized single-crystalline n-type CdS nanosheets via a chemical vapor deposition (CVD) method in a Cd-enriched ambient. The as-synthesized nanosheets are typically 40–100 nm thick, 10–300 µm wide, and up to several millimeters long. Using the nanosheets, we fabricated for the first time (to our knowledge), nano thin-film transistors (nano-TFTs) based on individual CdS nanosheets. A typical unit of such nanosheet TFTs has a high on–off ratio (∼1.7 ×109) and peak transconductance (∼14.1 µS), which to our knowledge are the best values reported so far for semiconductor nano-TFTs. In addition, we fabricated n-CdS nanosheet/p+-Si heterojunction light emitting diodes (LEDs) with a top electrode structure. This structure, where the n-type electrode is directly above the junction, has the advantage of a large active region and injection current favorable for high-efficiency electroluminescence (EL) and lasing. Room-temperature spectra of the LEDs consist of only an intense CdS band-edge emission peak (∼507.7 nm) with a full width at half-maximum of about 14 nm.

Magnetodielectric effect in CdS nanosheets grown within Na-4 mica

CdS nanosheets of thickness 0.6 nm were grown within the interlayer spaces of Na-4 mica. Magnetization measurements carried out in the temperature range 2–300 K showed the composites to have weak ferromagnetic-like properties even at room temperature. The saturation magnetization (MS) at room temperature was found to be higher than that reported for CdS nanoparticles. The higher value of MS may be ascribed to the presence of a large number defects in the present CdS system, due to a large surface to volume ratio in the nanosheets as compared to that of CdS nanoparticles. The nanocomposites exhibited a magnetodielectric effect with a dielectric constant change of 5.3% for a magnetic field of 0.5 T. This occurred due to a combination of magnetoresistance and Maxwell-Wagner effect as delineated in the model developed by Catalan.

Synthesis and characterization of single-crystal CdS nanosheet for high-speed photodetection

One-dimensional nanostructures have several unique advantages over bulk material and thin films, which can be exploited for high-speed photodetection. Furthermore, as bulk CdS has a high photosensitivity and quantum efficiency, there is considerable potential for the use of CdS nanostructures in advanced devices. In this study, single-crystal CdS nanosheets were grown by thermal evaporation and fully characterized to determine their potential for application in high-speed photodetectors. A high-quality nanosheet was confirmed to have a smooth surface with no extraneous particles and a strong orientation to the (110) plane of the wurtzite (hexagonal) phase of CdS. The Cd/S ratio was found to be nearly stoichiometric at 1.09. Photoluminescence measurement of a single-crystal CdS nanosheet showed a high emission intensity at a wavelength of 493nm. The current–voltage characteristics of the CdS nanosheet on Al thin film indicated an Ohmic contact in dark and under illumination by ambient, 365-nm, 405-nm, and 460-nm light. The light responsivity showed a peak at 460nm. Under 365-nm, 405-nm, and 460-nm chopped light, at a bias voltage of 1, 3, and 5V, the photocurrent rise and decay times were investigated. The device showed faster response times for 460-nm light. This fast response was attributed to the high quality of the single crystal, the absence of defect states, and the high surface/volume ratio. The device showed a high quantum efficiency of 22.3×103% when it was illuminated by 365-nm light under a bias of 5V; this efficiency increased to 36.3×103% and 40.5×103% when the device was illuminated by 405-nm and 460-nm light, respectively.

Nonlinear Two-Photon Photocurrent Spectroscopy of CdS Nanosheets

AbstractWe study the photocurrent from photoexcited charged carriers excited with lasers of energy both above and below the energy gap in CdS nanostructures. We observe non-linear photocurrents in CdS nanosheet devices in the metal-semiconductor-metal configuration with Schottky contacts for sub-band gap excitations. Analysis of two-photon absorption dominated photocurrents reveals a nonlinear coefficient of β = 2 cm/GW for these nanosheet devices, which is comparable to those of bulk CdS. We demonstrate the use of the photocurrent polarization measurements to determine the orientation of atoms in the nanosheet.

First-principles study on the electronic and magnetic properties of hydrogenated CdS nanosheets

Based on first-principles calculations, we study the electronic structures and magnetic properties of a two-dimensional CdS nanostructure upon hydrogenation adsorption. The results show that the hydrogen atoms can adsorb on Cd atoms within the graphenelike Cd layers with favorable formation energies, and the resulting semihydrogenated CdS systems are expected to show semimetallic properties with Curie temperatures above room temperature. These studies demonstrate that the decoration II-VI group semiconductor with hydrogen might be an efficient route for realizing the interesting long-range ferromagnetism in nanostructure materials.

Photocurrent spectroscopy of single CdS nanosheets: Valence band structure and two photon absorption

Photocurrent spectroscopy has been carried out on single CdS nanosheet devices in the metal-semiconductor-metal configuration with both Schottky and Ohmic contacts. Spatial imaging of the photocurrent shows that the photosensitive regions are localized at the reverse biased contact for Schottky type contacts and uniformly distributed throughout the nanosheet for Ohmic contacts. Photocurrent spectra show excitonic resonances at low temperatures corresponding to the A, B, C hole bands. Subband gap pulsed laser excitation reveals two-photon absorption dominated photocurrents consistent with a nonlinear coefficient of β=2 cm/GW for these nanosheet devices.

Synthesis of CdS hierarchical microspheres and their application in electrochemiluminescence sensing

CdS microspheres with hierarchical structures were prepared with sodium dodecylbenzenesulfonate (SDBS) as structure directing reagent. SEM analysis indicated that the CdS microspheres were constructed with CdS nanosheets, which were further built up with assembled CdS nanoparticles. Optical investigation showed that the obtained CdS microspheres have an UV–vis absorption at 445nm and photoluminescence (PL) emissions at 498 and 573nm. The CdS hierarchical microspheres were immobilized on a glassy carbon electrode and the electrochemiluminescence (ECL) properties were investigated. The first application to sensing H2O2 was studied based on ECL from CdS hierarchical microspheres. A linear relation between the ECL intensity and H2O2 concentration, I=2.4212+2.1713 c, was obtained with a detection limit of 1×10−8mol·L−1.

Two-Dimensional Single Crystal CdS Nanosheets: Synthesis and Properties

A one-dimensional (1D) to two-dimensional (2D) topotactic transformation has been proposed for the synthesis of 2D CdS nanosheets by promoting the lateral growth of 1D CdS nanobelts. The as-prepared nanosheets are single crystal with hexagonal structure, usually 20−100 nm in thickness, 20−100 μm in width, and up to several millimeters in length. CdS nanosheets show enhanced multiphonon responses and exciton−phonon couplings, which are correlated to their extraordinary anisotropic geometry. Photoluminescence measurement reveals three emission bands at about 1.61, 2.39, and 2.45 eV, which are attributed to the radiative transitions related to surface states, bandgap, and excitonic molecules, respectively; moreover, the formation of the surface states and the excitonic molecules are significantly enhanced due to the anisotropic geometry of the 2D nanosheets. CdS nanosheets show also pronounced photoconduction under visible light irradiation, indicating potentials in assembly of optoelectronic nanodevices.

Raman stress mapping of CdS nanosheets

Spatially resolved Raman scattering is used to probe the strain distribution in CdS nanosheets. We observe both significant strains and a significant strain gradient across the nanosheets. The magnitude of the strain suggests that electronic properties of the CdS nanosheets will be influenced as well. Using spatially resolved photoluminescence spectroscopy, we show that the band gap of the nanosheet experiences changes in the energy gap of 20 meV across the width of the nanosheet, which are consistent with our observations of the strain.

Morphological Evolution of CdS Nanowires to Nanosheets

We report on the formation mechanism of CdS nanosheets based upon extensive scanning electron microscopy and transmission electron microscopy experiments. Two different CdS nanowires were synthesized, whose axial directions are in parallel with [0001] or [0110]. The [0001]-nanowires sustained one-dimensional growth characteristics irrespective of reaction temperature and duration. On the other hand, we observed two-dimensional lateral growth in the [0110]-nanowires. We proposed a three-step process for the morphological evolution of CdS nanowires to nanosheets; (1) nucleation and growth of nanowires by vapor-liquid-solid mechanism, (2) side-branching of secondary nanowires perpendicular to the original nanowires and filling between them by vapor-solid mechanism, and finally (3) formation of nanosheets through the repetition of side-branching and filling processes.

Optical Properties of Single CdS Nanosheets

We study the optical properties of single CdS nanosheets using micro-photoluminescence and time-resolved photoluminescence (PL) techniques. At low temperature, we observe high quantum efficiency and strongly polarized PL emission, indicating high-quality single-crystal nanosheet growth. Photoluminescence from A and B exciton states is observed with different spatial distributions, suggesting the existence of a variation in strain in the nanosheets. Time-resolved photoluminescence measurements show the exciton lifetime to be ∼230 ps, longer than that of the near-band-edge emission, but shorter than the lifetime of defect-related emission in single CdS nanowires.

Polarized photoluminescence and time-resolved photoluminescence from single CdS nanosheets

We have utilized polarized low temperature photoluminescence (PL) to probe the electronic states and structural symmetries of individual CdS nanosheets. High resolution transmission electron microscopy measurements indicate highly crystalline material with different nanosheets exhibiting significant variations of the direction of the c axis, which are consistent with polarization measurements of PL from single CdS nanosheets. The quality of the nanosheets is reflected in measurements of exciton lifetimes of ∼200ps, a value significantly longer than observed for CdS nanowires whose diameter is the same as the thickness of these nanosheets, but shorter than that observed in bulk crystals.

Spatially resolved photoluminescence mapping of single CdS nanosheets

We have utilized spatially resolved low temperature photoluminescence to probe the electronic states and structural symmetries of individual single crystalline CdS nanosheets. Spatially resolved photoluminescence imaging of a single CdS nanosheet reveals a distinctive spectral variation across the nanosheet. We observe A-like exciton states which emit most strongly near the outer edges of the 5μm wide nanosheet, while B-like exciton states emit most strongly along the center region of the 30μm long axis of the nanosheet.

Self-Templated Synthesis of Nanoporous CdS Nanostructures for Highly Efficient Photocatalytic Hydrogen Production under Visible Light

Nanoporous CdS nanostructures, including nanosheets and hollow nanorods, have been prepared by a two-step aqueous route, which consists of a first precipitation of nanoporous Cd(OH)2 intermediates and a subsequent S2−/OH− ion-exchange conversion of the obtained Cd(OH)2 used as template either to nanoporous CdS nanosheets with sizes up to 60 nm and an average thickness of about 9 nm or to CdS hollow nanorods with lengths up to 30 nm and outer diameters in the range 7–14 nm. The obtained CdS nanostructures containing nanopores with diameters of ∼3 nm exhibit a very large BET surface area of about 112.8 m2 g−1. A very high hydrogen yield of about 4.1 mmol h−1 under visible light irradiation (λ ≥ 420 nm), corresponding to the highest apparent quantum yield of about 60.34% measured at 420 nm so far reported, has been attained over the obtained nanoporous CdS nanostructures loaded with monodisperse 3–5 nm Pt nanocrystals, which is due to an efficient charge separation, a fast transport of the photogenerated car...

Evolution of optical phonons in CdS nanowires, nanobelts, and nanosheets

We report Raman scattering from single and ensemble CdS nanowires, nanobelts, and nanosheets. The Raman spectra of nanobelts and nanosheets are notably different from those of nanowires, exhibiting a strong enhancement of the multiphonon response. Moreover, the first-order longitudinal optical (LO) phonon energy systematically increases with increasing lateral size from nanowires to nanobelts, and to nanosheets. These results suggest that the optical phonons in the CdS nanostructures are influenced by strain, crystallinity, and exciton-LO phonon coupling.

Synthesis and characterization of micrometer-sized polycrystalline CdS nanosheets

A simple two-step method is reported to fabricate micrometer-sized polycrystalline CdS nanosheets on large-scale. Metal Cd nanosheets were first grown via a thermal decomposition of CdS powder and then polycrystalline CdS nanosheets were obtained by subsequent sulfuration. Investigation results demonstrate that the polycrystalline CdS nanosheets have dimensions of several tens of microns and an average thickness of about 80 nm with hexagonal phase structure. TEM image reveals that the nanosheet is composed of many randomly oriented polycrystalline grains. Temperature and gas flow are supposed as the determining factor for the formation of polycrystalline structure.