CdSe Nanowires Research Articles

Ultralong Homogeneously Alloyed CdSexS1‐x Nanowires with Highly Polarized and Color‐Tunable Emissions

Colloidal semiconductor nanowires are attractive materials with polarized light absorption and emissions. Solution synthesis of ultralong (several tens of micro­meters) colloidal CdSexS1‐x nanowires with homogeneously alloyed composition through a solution–liquid–solid (SLS) method are reported. By varying the ratio of Se/S, the optical band and their corresponding photoluminescence spectra of the CdSexS1‐x nanowires can be finely tuned from 508 nm (ca. 2.45 eV) to 628 nm (ca. 2.0 eV). In comparison with the low quantum yields of pure CdS and CdSe nanowires (less than 0.1%), alloyed CdSexS1‐x nanowires exhibit enhanced bright photoluminescence emission with a narrow spectral width of 25–31 nm and moderate quantum yields up to ca. 8.8%. Moreover, the randomly oriented ensembles on glass wafers show strong polarized emission with polarization values of ca. 0.6 due to the inherent larger aspect ratio. The combination of these characteristics in alloyed nanowires sets the basis for the spectroscopic study of 1D excitons, as well as the fabrication of nanowire‐based polarized photonic and optoelectronic devices.

Modifications in structural and electrical properties of gamma irradiated CdSe nanowires

Semiconducting nanowires have gained huge interest of researchers due to their interesting properties and potential application in the field of nanoelectronics and optoelectronics. Present study reports the effect of gamma (γ) irradiation on structural and electrical properties of CdSe nanowires. These nanowires were synthesized by a template assisted electrodeposition method which provides uniform and standing nanowires of cylindrical shape. These synthesized nanowires were exposed to gamma rays using a 60Co source. Before and after irradiation, the nanowires were characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM), Energy dispersive X-ray (EDX) and Keithley 2400 series source meter to study their structural and electrical properties. XRD patterns of irradiated samples showed increase in grain size with radiation dose and increase in the peak intensity was observed corresponding to different planes of hexagonal CdSe structure. SEM images confirmed the cylindrical morphology of nanowires and EDX analysis revealed the stoichiometric formation of CdSe. The electrical properties from I–V characteristics showed an enhancement in the electrical conductivity of the nanowires with radiation dose and applied potential. This modification in electrical conductivity may be attributed to decrease in grain boundary scattering of charge carriers.

Single Nanostructure Extinction Spectroscopy

Molecules and nanoparticles exhibit unique properties such as emission intermittency (i.e. blinking) and spectral wandering which are only observable at the single molecule level. Until recently, single molecule detection has mainly been restricted to fluorescent entities. Now techniques such as photothermal imaging and spatial modulation microscopy allow single molecules and nanoparticles to be detected, regardless of their ability to emit light. This opens up the possibility of discovering new absorptive properties and even photophysics that have previously been masked within ensemble measurements. In this talk, I describe the use of these direct absorption techniques for two case studies. The first is on CdSe nanowires, a material that has potential uses in renewable energy applications. Through direct absorption measurements, we obtain the first absorption size series of any nanowire system and reveal the complex interplay between competing effects such as confinement and dielectric contrast, which determine the optical response of the wires. In a second case study, we focus on graphene oxide, an important precursor in the production of chemically-derived graphene, another important material for renewable energy applications. We show, for the first time, the direct observation of laser-induced, single layer GO reduction through correlated changes to its absorption and emission. Acquired absorption/emission movies illustrate the initial stages of single layer GO reduction, its transition to reduced-GO (rGO) as well as its subsequent decomposition upon prolonged laser illumination. These studies reveal GO’s photoreduction life cycle and through it native GO/rGO absorption coefficients as well as their intrasheet distributions and spatial heterogeneities. Beyond these two case studies, the demonstrated single nanostructure absorption technique offers exciting possibilities for directly interrogating other systems that are not necessarily fluorescent, revealing insights beyond simple ensemble averages.

Nanowire-Functionalized Cotton Textiles

We show the general functionalization of cotton fabrics using solution-synthesized CdSe and CdTe nanowires (NWs). Conformal coatings onto individual cotton fibers have been achieved through various physical and chemical approaches. Some involve the electrostatic attraction of NWs to cotton charged positively with a Van de Graaff generator or via 2,3-epoxypropyltrimethylammonium chloride treatments. Resulting NW-functionalized textiles consist of dense, conformal coatings and have been characterized for their UV-visible absorption as well as Raman activity. We demonstrate potential uses of these functionalized textiles through two proof-of-concept applications. The first entails barcoding cotton using the unique Raman signature of the NWs. We also demonstrate the surface-enhancement of their Raman signatures using codeposited Au. A second demonstration takes advantage of the photoconductive nature of semiconductor NWs to create cotton-based photodetectors. Apart from these illustrations, NW-functionalized cotton textiles may possess other uses in the realm of medical, anticounterfeiting, and photocatalytic applications.

Size and orientation dependent melting properties and behavior of wurtzite CdSe nanowires

The thermal stability and melting behavior of wurtzite CdSe nanowires have been studied by molecular dynamic simulation. The melting temperatures of CdSe nanowires show clear dependence on nanowires sizes and orientations. The overall melting temperatures of CdSe nanowires are much lower than that of bulk and will increase with increasing size. Compared with other oriented nanowires, the 〈0001〉 nanowire possesses a better thermal and structural stability, which helps to explain why wurtzite type CdSe nanowires prefer to grow in 〈0001〉 orientation during experiments. It is also found that the premelting of 〈0001〉 and 〈101¯0〉 nanowires starts from the surface atoms, while the premelting of 〈21¯1¯0〉 nanowire starts from the interior atoms. This unique melting behavior for the 〈21¯1¯0〉 nanowire may be related to the interesting structure reconstruction during the heating process.

Electrochemical synthesis of CdSe/CdTe nanowires for hybrid photovoltaic structures

ABSTRACTAdvanced electrochemical technique was elaborated to fabricate self-organized CdSe nanowire structures from aqueous electrolytes on ITO coated glass substrates. We have recently been demonstrated successful electrochemical formation of free-assistent CdSe nanowire structures with diameter around 30 nm. This work has extended our previous research of electrodeposition of Cd chalcogenide (CdSe, CdS) nanowires to formation of core-shell CdSe/CdTe photosensitive nanowire structures. CdSe nanowire structures were synthesized potentiostatically from an acidic solution of H2SeO3 and CdCl2 at room temperature. Then the CdSe (core) nanowires were further passivated with CdTe (shell) thin film by method of electrochemical deposition from acidic solution of H2TeO3 and CdCl2. The effect of interfacial passivation with CdTe layer on the performance of the prepared photovoltaic structures was investigated and special account was paid to the morphology, composition and photovoltaic properties of obtained CdSe/CdTe nano-layers. It should be noted, that electrically conductive polymer photoabsorbers (poly (3-octylthiophene) etc.) were applied successfully for preparation of high work-function ohmic contact-sensitizer layer to CdTe shells. The electrodeposition and spin-casting techniques were applied step-by-step to prepare complete hybrid photovoltaic structures.

Novel graphene–oxide–semiconductor nanowire phototransistors

Novel graphene–oxide–semiconductor (GOS) nanowire phototransistors have been fabricated for the first time. Monolayer graphene, high-κ HfO2, and CdSe nanowire (NW) were used as transparent top-gate, gate dielectric, and conductive channel, respectively. Electric measurements of the devices reveal a clear field effect. In the dark, the on/off ratio, threshold voltage, subthreshold swing, and peak transconductance are about 4.9 × 106, −1.5 V, 120 mV dec−1, and 2.3 μS, respectively. The photo-response characterization demonstrates that the gate voltage has a remarkable modulation effect on the responsivity of the phototransistors. Under 633 nm light illumination, the responsivity, gain, and specific detectivity can be as high as 1.06 × 107 A W−1, 1.93 × 107, and 9.68 × 1015 Jones, respectively. To the best of our knowledge, these values are among the highest reported so far for NW-based photodetectors. Our results demonstrate that GOS NW phototransistors promise a potential application in weak light detecting, which is highly desired in future optical communication or nanoscale integrated optical circuits.

Magneto-optical spectrum and the effective excitonic Zeeman splitting energies of Mn and Co-doped CdSe nanowires

The electronic structure of Mn and Co-doped CdSe nanowires are calculated based on the six-band k·p effective-mass theory. Through the calculation, it is found that the splitting energies of the degenerate hole states in Mn-doped CdSe nanowires are larger than that in Co-doped CdSe nanowires when the concentration of these two kinds of magnetic ions is the same. In order to analysis the magneto-optical spectrum of Mn and Co-doped CdSe nanowires, the four lowest electron states and the four highest hole states are sorted when the magnetic field is applied, and the 10 lowest optical transitions between the conduction subbands and the valence subbands at the Γ point in Mn and Co-doped CdSe nanowires are shown in the paper, it is found that the order of the optical transitions at the Γ point almost do not change although two different kinds of magnetic ions are doped in CdSe nanowires. Finally, the effective excitonic Zeeman splitting energies at the Γ point are found to increase almost linearly with the increase of the concentration of the magnetic ions and the magnetic field; meanwhile, the giant positive effective excitonic g factors in Mn and Co-doped CdSe nanowires are predicted based on our theoretical calculation.

Controlled Electrodeposition of Bismuth Nanocatalysts for the Solution–Liquid–Solid Synthesis of CdSe Nanowires on Transparent Conductive Substrates

Semiconductor nanowires (NWs) composed of cadmium selenide (CdSe) have been directly grown on transparent conductive substrates via the solution-liquid-solid (SLS) approach using electrodeposited bismuth nanoparticles (Bi NPs) as catalyst. Bi NPs were fabricated on indium tin oxide (ITO) surfaces from a bismuth trichloride solution using potentiostatic double-pulse techniques. The size and density of electrodeposited Bi NPs were controlled by the pulse parameters. Since the NW diameter is governed by the dimension of the Bi catalyst, the electrodeposition is a reliable method to synthesize nanowires directly on substrates with a desired size and density. We show that the density can be adjusted from individual NWs on several square micrometer to very dense NW networks. The diameter can be controlled between thick nanowires above 100 nm to very thin NW of 7 nm in diameter, which is well below the respective exciton dimension. Hence, especially the thinnest NWs exhibit diameter-dependent photoluminescence energies as a result of quantum confinement effects in the radial dimension.

Sequentially Layered CdSe/CdS Nanowire Architecture for Improved Nanowire Solar Cell Performance

The power conversion efficiency of semiconductor nanowire (NW) based solar cells as compared to quantum dot solar cell (QDSC) has remained lower, and efforts to improve the photovoltaic performance of semiconductor NWs continue. We have now succeeded in using a layered architecture of CdS and CdSe NWs for improving the photovoltaic performance of nanowire solar cell (NWSC). The photoanode designed with sequentially deposited films of CdSe and CdS NWs delivered a power conversion efficiency of 1%. This efficiency of CdSe/CdS composite is an order of magnitude improvement over single nanowire system (CdS or CdSe) based solar cell. The improvement seen in the CdSe/CdS composite film is attributed to charge rectification and improvement of electron and hole separation and transport in the opposite direction. Impedance spectroscopy demonstrates the beneficial effect of type II structure in CdSe/CdS sequential deposition through lower transport resistance, which remains a dominating effect in dictating the over...

Supercontinuum spatial modulation spectroscopy: Detection and noise limitations

Supercontinuum spatial modulation spectroscopy is a facile tool for conducting single molecule/particle extinction spectroscopy throughout the visible and near infrared (420-1100 nm). The technique's capabilities are benchmarked using individual Au nanoparticles (NPs) as a standard since they are well studied and display a prominent plasmon resonance in the visible. Extinction spectra of individual Au NPs with diameters (d) ranging from d ~ 8 to 40 nm are resolved with extinction cross sections (σ(ext)) of σ(ext) ~ 1 × 10(-13)-1 × 10(-11) cm(2). Corresponding signal-to-noise ratios range from ~30 to ~1400. The technique's limit of detection is determined to be 4.3 × 10(-14) cm(2) (4.3 nm(2)). To showcase supercontinuum spatial modulation spectroscopy's broader applicability, extinction spectra are acquired for other model systems, such as individual single-walled carbon nanotubes (SWCNTs) and CdSe nanowires. We show for the first time extinction spectra of individual (8,3) and (6,5) SWCNTs. For both chiralities, their E11 [(8,3) 1.30 eV (952 nm); (6,5) 1.26 eV (986 nm)] and E22 [(8,3) 1.86 eV (667 nm); (6,5) 2.19 eV (567 nm)] excitonic resonances are seen with corresponding cross sections of σ(ext) ~ 10(-13) cm(2) μm(-1).

Hybrid Photon-Plasmon Nanowire Lasers

Metallic and plasmonic nanolasers have attracted growing interest recently. Plasmonic lasers demonstrated so far operate in hybrid photon-plasmon modes in transverse dimensions, rendering it impossible to separate photonic from plasmonic components. Thus only the far-field photonic component can be measured and utilized directly. But spatially separated plasmon modes are highly desired for applications including high-efficiency coupling of single-photon emitters and ultrasensitivity optical sensing. Here, we report a nanowire (NW) laser that offers subdiffraction-limited beam size and spatially separated plasmon cavity modes. By near-field coupling a high-gain CdSe NW and a 100 nm diameter Ag NW, we demonstrate a hybrid photon-plasmon laser operating at 723 nm wavelength at room temperature, with a plasmon mode area of 0.008λ(2). This device simultaneously provides spatially separated photonic far-field output and highly localized coherent plasmon modes, which may open up new avenues in the fields of integrated nanophotonic circuits, biosensing, and quantum information processing.

Solution-Based Stoichiometric Control over Charge Transport in Nanocrystalline CdSe Devices

Using colloidal CdSe nanowire (NW) field-effect transistors (FETs), we demonstrated the dependence of carrier transport on surface stoichiometry by chemically manipulating the atomic composition of the NW surface. A mild, room-temperature, wet-chemical process was devised to introduce cadmium, selenium, or sulfur adatoms at the surface of the NWs in completed devices. Changes in surface composition were tested for by energy dispersive spectroscopy and inductively coupled plasma-atomic emission spectroscopy and through the use of the vibrational reporter thiocyanate. We found that treatment with cadmium acetate enhances electron currents, while treatment with sodium selenide or sodium sulfide suppressed them. The efficacy of doping CdSe NWs through subsequent thermal diffusion of indium was highly dependent on the surface composition. While selenium-enriched CdSe NW FETs were characterized by little to no electron currents, when combined with indium, they yielded semimetallic devices. Sulfur-enriched, indium-doped devices also displayed dramatically enhanced electron currents, but to a lesser extent than selenium and formed FETs with desirable ION/IOFF >10(6). The atomic specificity of the electronic behavior with different surface chalcogens suggested indium was bound to chalcogens at the NW surface, indicating commonalities with and implications for indium-containing CdSe nanocrystal films. Low temperature measurements of indium-doped CdSe NW FETs showed no evidence of impurity scattering, further supporting the existence of an indium-chalcogen interaction at the surface rather than in the core of the NW.

Electroluminescent, Polycrystalline Cadmium Selenide Nanowire Arrays

Electroluminescence (EL) from nanocrystalline CdSe (nc-CdSe) nanowire arrays is reported. The n-type, nc-CdSe nanowires, 400-450 nm in width and 60 nm in thickness, were synthesized using lithographically patterned nanowire electrodeposition, and metal-semiconductor-metal (M-S-M) devices were prepared by the evaporation of two gold contacts spaced by either 0.6 or 5 μm. These M-S-M devices showed symmetrical current voltage curves characterized by currents that increased exponentially with applied voltage bias. As the applied biased was increased, an increasing number of nanowires within the array "turned on", culminating in EL emission from 30 to 50% of these nanowires at applied voltages of 25-30 V. The spectrum of the emitted light was broad and centered at 770 nm, close to the 1.74 eV (712 nm) band gap of CdSe. EL light emission occurred with an external quantum efficiency of 4 × 10(-6) for devices with a 0.60 μm gap between the gold contacts and 0.5 × 10(-6) for a 5 μm gap-values similar to those reported for M-S-M devices constructed from single-crystalline CdSe nanowires. Kelvin probe force microscopy of 5 μm nc-CdSe nanowire arrays showed pronounced electric fields at the gold electrical contacts, coinciding with the location of strongest EL light emission in these devices. This electric field is implicated in the Poole-Frenkel minority carrier emission and recombination mechanism proposed to account for EL light emission in most of the devices that were investigated.

Size and temperature dependence of the tensile mechanical properties of zinc blende CdSe nanowires

Abstract The effect of size and temperature on the tensile mechanical properties of zinc blende CdSe nanowires is investigated by all atoms molecular dynamic simulation. We found the ultimate tensile strength and Youngʼs modulus will decrease as the temperature and size of the nanowire increase. The size and temperature dependence are mainly attributed to surface effect and thermally elongation effect. High reversibility of tensile behavior will make zinc blende CdSe nanowires suitable for building efficient nanodevices.

Acoustic breathing mode frequencies in cylinders, cylindrical shells and composite cylinders of general anisotropic crystals: Application to nanowires

We present here a study of the acoustic breathing modes for infinitely long cylinders, cylindrical shells and composite cylinders of general anisotropic crystals. We assume cylindrical anisotropy for the systems studied. We obtain expressions in closed form for their frequencies in the case of cylinders and cylindrical shells, valid for any anisotropic material, thus including up to 21 independent elastic constants. In the case of the lowest breathing mode of a thin cylindrical shell we obtain a simple analytical formula. This can be used to obtain a first estimate of the breathing mode frequency in nanotubes for any material. In the case of core–shell and composite cylinders we obtain the expressions for the secular determinant. We calculate the frequencies of the lowest acoustic breathing mode of Au, CdSe, InAs, GaAs, Ag and Bi nanowires obtained recently by different experimental groups. We also present results for the acoustic breathing modes of Au/Ag and ZnS/SiO2 core–shell nanowires produced recently.

A study of the size-dependent elastic properties of CdSe nanowires

The size dependence of elastic properties of CdSe nanowire in [0001] direction is investigated by all atoms molecular dynamic simulation. We found that both the Young’s modulus and Poisson ratio of the nanowires decrease with increasing radius, and the Young’s moduli of the nanowires studied here are significantly higher than that of bulk material. The physical origin of the size dependence can be explained by the factor that high surface stiffness effect decays with decreasing surface to volume ratio. The results of VDOS show that the high frequency part will enhance as the nanowire radius decreases, this trend is consistent with that of the Young’s modulus of the nanowires.

A new route to produce efficient surface-enhanced Raman spectroscopy substrates: gold-decorated CdSe nanowires

Surface-enhanced Raman spectroscopy is a popular tool for the detection of extremely small quantities of target molecules. Au nanoparticles have been very successful in this respect due to local enhancement of the light intensity caused by their plasmon resonance. Furthermore, Au nanoparticles are biocompatible, and target substances can be easily attached to their surface. Here, we demonstrate that Au-decorated CdSe nanowires when employed as SERS substrates lead to an enhancement as large as 105 with respect to the flat Au surfaces. In the case of hybrid metal–CdSe nanowires, the Au nucleates preferably on lattice defects at the lateral facets of the nanowires, which leads to a homogeneous distribution of Au nanoparticles on the nanowire, and to an efficient quenching of the nanowire luminescence. Moreover, the size of the Au nanoparticles can be well controlled via the AuCl3 concentration in the fabrication process. We demonstrate the effectiveness of our SERS substrates with two target substances, namely, cresyl-violet and rhodamine-6G. Au-decorated nanowires can be easily fabricated in large quantities at low cost by wet-chemical synthesis. Furthermore, their deposition onto various substrates, as well as the functionalization of these wires with the target substances, is as straightforward as with the traditional markers.

Ultrafast Spectroscopy of Quantum Confined States in a Single CdSe Nanowire

We measure for the first time transient absorption spectra of individual CdSe nanowires with about 10 nm diameter. Confinement of the carrier wave functions leads to discrete states which can be described by a six-band effective mass model. Combining transient absorption and luminescence spectroscopy allows us to track the excitation dynamics in the visible and near-infrared spectral range. About 10% of all absorbed photons lead to an excitation of the lowest energy state. Of these excitations, less than 1% lead to a photon in the optical far-field. Almost all emission is reabsorbed by other parts of the nanowire. These findings might explain the low overall quantum efficiency of CdSe nanowires.

A novel and high yield synthesis of CdSe nanowires

CdSe nanowires (CdSe-NWs) at large scale were obtained through a simple and clean method. The reaction was carried out without complexing agents which often facilitate the preferential nanostructure growth. In this work, CdSe-NWs with lineal and zigzag shape around 40 and 130 nm in diameter with a wurtzite-type structure were synthesized. Quantitative EDX analysis indicated the stoichiometric formation of CdSe, while both HRTEM and HAADF-STEM analysis ruled out the presence of other phases such as CdO or nanotubes formation, respectively. Diameter and length of the nanowires varied with the reaction time and temperature. This synthesis method makes the nanostructures purification process easier and also is non-toxic and its high conversion make it an efficient method for obtaining CdSe-NWs.