Conduction Band Of CdSe Research Articles

Construction of CdSe-AuPd quantum dot 0D/0D hybrid photocatalysts: charge transfer dynamic study with electrochemical analysis for improved photocatalytic activity.

The integration of semiconductor quantum dots and noble metal nanoparticles can efficiently couple numerous effects corresponding to the individual domains of the hybrid system for a variety of applications. Herein, we establish a direct correlation between the electronic band structure and optical band gap of monometallic and bimetallic alloy nanoparticle decorated CdSe quantum dots, which in turn regulate the charge shuttling dynamics in a quantum dot hybrid (QDH) system. Directly coupled Au, Pd, AuPd, and CdSe QDHs were prepared via a simple fabrication technique. The photoluminescence intensity of the QDHs was quenched compared to that of CdSe quantum dots with a maximumally diminished CdSe-AuPd system. Broadening of the absorbance peak along with a blue shift for QDHs confirm the interaction of the energy levels of the QDs and metal domains. AuPd decorated CdSe QDs demonstrate enhanced photocatalytic activity compared to their monometallic counterparts, which has made them interesting catalysts reported for the first time. Lifetime decay measurements, which isolated the individual charge-transfer steps, showed that a maximum amount of photoexcitons can be separated by bimetallic alloy decoration compared to monometallic ones. Cyclic voltammetry results offer insight into the change in the conduction band edge energy position for both monometallic and bimetallic incorporating semiconductor hybrid systems. Our findings reveal that photoexcited semiconductor quantum dots undergo charge equilibration when the QDs are in contact with metallic domains, influencing the shifting of the conduction band energy level of the hybrid to a more negative potential, and this is a maximum for the CdSe-AuPd hybrid, resulting in the best photocatalytic activity. Shuttling of electrons around the conduction band of CdSe and the Fermi level of the metallic domains is the main deciding factor for an efficient photocatalyst hybrid system.

Gold nanoparticle on semiconductor quantum dot: Do surface ligands influence Fermi level equilibration.

Semiconductor-metal heterojunction nanostructures possess an ability to store electrons upon photoexcitation through Fermi level equilibration. The unique role of capping ligands in modulating the equilibration of Fermi level in CdSe-Au heteronanostructures is explored by taking alkyl thiols and alkyl amines as examples. Alkyl thiol having its highest occupied molecular orbital (HOMO) above the valence band of the heterojunction nanostructure inhibits the exciton recombination by scavenging the photogenerated hole. This leads to the elevation in the Fermi level of Au and equilibration with the conduction band of CdSe. The Fermi level equilibrated electrons are further transferred to an acceptor molecule such as methyl viologen, demonstrating the potential of heterojunction nanostructures capped with hole accepting ligands for charge transport application in photovoltaics. In contrast, alkyl amine being a non-hole acceptor ligand with its HOMO placed below its valence band promotes rapid Au mediated exciton recombination, limiting its usefulness in charge transport application. Thus, the energetics of ligands on heterojunction nanostructures plays a decisive role in Fermi level equilibration.

Electron Trap to Electron Storage Center in Specially Aligned Mn-Doped CdSe d-Dot: A Step Forward in the Design of Higher Efficient Quantum-Dot Solar Cell.

Specially aligned surface-accumulated Mn-doped CdSe (MnCdSe) quantum dots (QDs) have been synthesized to study the effect of dopant atom on charge-carrier dynamics in QD materials. EPR studies suggest that the (4)T1 state of Mn(2+) lies above the conduction band of CdSe, and as a result no Mn-luminescence was observed from MnCdSe. Femtosecond transient absorption studies suggest that Mn atom introduces structural defects in surface-doped CdSe, which acts as electron trap center in doped QD for the photoexcited electron. Bromo-pyrogallol red (Br-PGR) were found to form strong charge-trasfer complex with both CdSe and MnCdSe QDs. Charge separation in both the CdSe/Br-PGR and MnCdSe/Br-PGR composites was found to take place in three different pathways by transferring the photoexcited hole of CdSe/MnCdSe QDs to Br-PGR, electron injection from photoexcited Br-PGR to the QDs, and direct electron transfer from the HOMO of Br-PGR to the conduction band of both the QDs. Hole-transfer dynamics are found to be quite similar (∼1.1 to 1.3 ps) for both of the systems and found to be independent of Mn doping. However, charge recombination dynamics was found to be much slower in the MnCdSe/Br-PGR system as compared with that in the CdSe/Br-PGR system, which confirms that the Mn dopant act as the electron storage center. As a consequence, the MnCdSe/Br-PGR system can be used as a better super sensitizer in quantum-dot-sensitized solar cell to increase efficiency further.

Nickel Catalyst Boosts Solar Hydrogen Generation of CdSe Nanocrystals

With the rapid growth of global population and energy demand, exploring renewable and sustainable energy from natural resources such as sunlight is critical. Although solar energy is a promising solution for future energy demand, the solar irradiation on earth is an intermittent source of energy; and, therefore, it requires an efficient and cost effective method to store the excess energy. One of the most attractive approaches is to convert solar energy into the chemical fuels such as hydrogen. For instance, nature is capable of transforming solar energy into carbohydrates through photosynthesis. Likewise, solar energy can be stored in the form of hydrogen through artificial photosynthesis (solar water splitting). Hydrogen gas is an environmentally attractive chemical fuel with high energy density of 140 MJKg . Artificial photosynthesis is a chemical process that involves three critical steps: light harvesting; charge separation; as well as charge transport and transfer. Chromophores, such as dye molecules, or semiconductor materials can be employed as the light-absorbing medium. Photo-bleaching of the dye, however, is a major limitation for their application in solar conversion devices. In comparison with organic or organometallic dye molecules, semiconductor nanocrystals exhibit excellent photostability. Additionally, the small dimension of nanocrystals provides large surface area and short diffusion length for charge carriers. Among different semiconductors, CdSe nanocrystals hold great promise as efficient chromophores for solar water splitting. Bulk CdSe has a favorable bandgap of 1.7 eV for visible light absorption. The conduction band of CdSe is more negative than proton reduction potential, making it suitable for photo-generation of hydrogen. Previous studies have shown that CdSe nanoribbons and nanocrystals exhibit photoactivity for photochemical hydrogen evolution upon irradiation with ultraviolet and visible light in aqueous solution containing sacrificial electron donors such as Na2S/Na2SO3 solution. [3,4] Yet, the hydrogen photo-generation rate was modest, with turnover frequency of 2, which could be attributed to the large overpotential for proton reduction on the CdSe surface as well as the slow interfacial electron transfer between CdSe nanocrystals and electrolyte. Additionally, the relatively poor electrochemical stability of CdSe was another concern. CdSe can be easily oxidized by oxygen unless sacrificial hole scavengers such as S are added in the electrolyte solution. Recently, Han and co-workers at University of Rochester reported a robust and highly active Ni catalyst modified CdSe nanocrystal system for photo-generation of hydrogen in aqueous solution, with a remarkable turnover frequency of >600000 and an extraordinary longevity for hydrogen generation. The catalytic mechanism of Ni-modified CdSe (denoted as Ni-CdSe) nanocrystals is illustrated in Figure 1a. CdSe nanocrystals capped with dihydrolipoic acid (DHLA) were synthesized by a hot injection method and followed ligand exchange method. Aqueous solution containing 1.0m ascorbic acid as sacrificial electron donor was used as electrolyte solution. Ni salt [Ni(NO3)2,] was added as photocatalyst for proton reduction. Upon illumination, photo-excited electrons in CdSe nanocrystals shuttle to Ni catalyst and reduce protons to hydrogen; and photo-excited holes oxidize ascorbic acid to regenerate protons. Figure 1b shows the performance of CdSe nanocrystals for hydrogen production over time in the presence and absence of Ni catalyst. The linear profiles indicate that the hydrogen generation rates are constant for over 360 h, proving that the photoactivities of CdSe nanocrystals are extremely stable. Significantly, the Ni-CdSe nanocrystals boost the rate of hydrogen generation by more than an order of magnitude compared to bare CdSe nanocrystals, indicating Ni is excellent catalyst that facilitates the electron transfer from CdSe to electrolyte for proton reduction. In optimized conditions, Ni-CdSe system achieved a remarkable turnover number over 600000 moles of H2 per mole of catalyst. A quantum yield of 36% was obtained at the irradiation wavelength of 520 nm in aqueous solution. Even higher quantum yield of 60% at 520 nm was achieved in a 1:1 water-ethanol solvent, which is a benchmark value for an aqueous system. Four key innovations were demonstrated in the work of Han et al. , which presents a scientific breakthrough in the development of catalyst for photo-generation of hydrogen. First, the reported Ni-CdSe nanocrystal system showed outstanding photocatalytic performance with turnover number over 600000 moles of H2 per mole of catalyst, which is substantially improved, compared to the previously reported bare CdSe nanocrystals as well as hydrogen evolution catalyst (HER) modified CdSe systems. Second, the Ni-CdSe system demonstrated excellent longevity. Previous studies have showed that the photoactivity of CdSe for hydrogen generation can only last for less than 20 h. Significantly, the work of Han et al. proved that the Ni-CdSe system can be operated in aqueous solution without obvious photoactivity loss for two weeks, [a] G. Wang, Prof. Y. Li Department of Chemistry and Biochemistry University of California, Santa Cruz California 95064 (United States) Fax: (+1)831-459-2935 E-mail : yli@chemistry.ucsc.edu

Direct evidence of type II band alignment in nanoscale P3HT/CdSe heterostructures

Due to inherent advantages of both constituent materials, organic/inorganic hybridcomposites have attracted increasing attention. One of the fundamental issues needed to beresolved is their band alignment, which governs most of the electrical and opticalproperties. Here, we report the investigation of optical transition in poly(3-hexylthiophene)(P3HT)/CdSe nano-composites (NCs). It is found that the relaxation dynamics ofphoto-carriers in NCs is dominated by charge separation effects. Based on the bandbending effect and the quantum confinement energy of electrons in the conduction band ofCdSe quantum dots, we provide direct evidence of type II band alignment in P3HT/CdSeNCs. The establishment of a type II transition in NCs is very useful for the future design ofefficient optoelectronic devices based on conjugated polymer/semiconductor hybridsystems.

HYBRID NANOMATERIALS FOR MULTI-SPECTRAL INFRARED PHOTODETECTION

Quantum dot infrared photodetectors (QDIPs) using quantum dots (QDs) grown by strained-layer epitaxy have demonstrated low dark current, multi-spectral response, high operating temperature, and infrared (IR) imaging. However, achieving near room-temperature, multi-spectral operation is a challenge due to randomness in QD properties. The ability to control dopant incorporation is important since charge carrier occupation influences dark current and IR spectral response. In this work, dopant incorporation is investigated in two classes of QDs; epitaxial InAs/GaAs QDs and CdSe colloidal QDs (CQDs) embedded in MEH-PPV conducting polymers. The long-term goal of this work is to combine these hybrid nanomaterials in a single device heterostructure to enable multi-spectral IR photodetection. Two important results towards this goal are discussed. First, by temperature-dependent dark current-voltage and polarization-dependent Fourier transform IR spectroscopy measurements in InAs/GaAs QDIPs featuring different doping schemes, we have provided experimental evidence for the important contribution of thermally-activated, defect-assisted, sequential resonant tunneling. Second, the enhanced quantum confinement and electron localization in the conduction band of CdSe / MEH-PPV nanocomposites enable intraband transitions in the mid-IR at room temperature. Further, by controlling the semiconductor substrate material, doping type, and doping level on which these nanocomposites are deposited, the intraband IR response can be tuned.

Optical and electrical characterization of cadmium selenide doped with cobalt

The optical and electrical properties of Co 2+ ions in CdSe have been investigated. Absorption, photoluminescence, electron paramagnetic resonance, and Hall measurements were used to characterize a cobalt-doped (1×10 19 cm −3) single crystal. Infrared absorption and emission spectra associated with transitions between the 4 A 2( F) ground state and the 4 T 1( F) and 4 T 2( F) excited states are described. At 10 K, spin–orbit splittings result in three structured absorption bands associated with the 4 A 2( F) to 4 T 1( F) transition having zero-phonon lines at 4926, 5101, and 5724 cm −1. The 4 A 2( F) to 4 T 2( F) transition shows two zero-phonon lines at 2874 and 3286 cm −1, also accompanied by vibronic structure. Intrinsic lattice modes explain most of the sharp-line structure observed at low temperature, except for a subset of peaks where local modes (25–30 cm −1) are invoked. Using below-band-gap light, selective excitation allows detection of the 4 T 1( F) to 4 A 2( F) recombination at liquid-helium temperatures. The activation of free carriers in our n-type material containing shallow donors is affected by the presence of cobalt, and we find the Co +/++ level to be about 34 meV below the conduction band of CdSe.

The Resonant States of the Ti(d2) Impurity in CdSe

AbstractOptical and electrical investigations show that Ti introduces a shallow donor in CdSe. The character of this donor is determined by the well localized 3d2shell of the Ti impurity substitutional on cadmium sites. The energy position of the Ti(d2) level depends on temperature and hydrostatic pressure. It is demonstrated that at high temperatures the Ti (d2) ground state becomes degenerate with the conduction band of CdSe. It is also shown that the excited 3T1 (F) and 3T1(P) states of the Ti (d2) are resonant states. Autoionization of the excited Ti (d2) impurity is observed in photoconductivity measurements.