Quantum Dot Hybrid Research Articles

Multiheterojunction Phototransistors Based on Graphene–PbSe Quantum Dot Hybrids

Graphene-semiconductor quantum dot (QD) hybrid field effect phototransistors (FEpTs) have attracted much interest due to their ultrahigh gain and responsivity in photo detection. However, most reported results are based on single-layer heterojunction, and the multiheterojunction FEpTs are often ignored. Here, we design two typical multiheterojunction FEpTs based on graphene–PbSe quantum dot (QD) hybrids, including QD at the bottom layer (QD-bottom) and graphene at the bottom layer (G-bottom) FEpTs. Through a comparative study, G-bottom FEpTs showed a multisaturation behavior due to the multigraphene layer effect, which was absent in the QD-bottom FEpTs. The mobilities for electrons and holes were μE = 147 cm2 V–1 s–1 and μE = 137 cm2 V–1 s–1 in the G-bottom FEpTs and μE = 14 cm2 V–1 s–1 and μE = 59 cm2 V–1 s–1 in the QD-bottom FEpTs. Higher responsivity (∼106 A W–1) and faster response rate were both achieved by the G-bottom FEpTs. All of the advantages in G-bottom FEpTs were attributed to the back-gate e...

A study of energy absorption rate in a quantum dot and metallic nanosphere hybrid system

We have studied energy absorption rate in a quantum dot-metallic nanosphere system embedded on a dielectric substrate. We applied a control field to induce dipole moments in the quantum dot and the metal nanosphere, and monitored the energy absorption using a probe field. These external fields induce dipole moments in the metal nanosphere and the quantum dot, and these two structures interact with one another via the dipole–dipole interaction. The density matrix method was used to evaluate the absorption, indicating that it can be shifted by moving the metal nanosphere close to the quantum dot. Also, absorption efficiency can either be quenched or enhanced by the addition of a metal nanosphere. This hybrid system can be used to create ultrafast switching and sensing nanodevices.

Significantly enhanced electrochemical performance of lithium titanate anode for lithium ion battery by the hybrid of nitrogen and sulfur co-doped graphene quantum dots

ABSTRACT The paper reported a facile synthesis of lithium titanate/nitrogen and sulfur co-doped graphene quantum dots(LTO/N,S-GQDs). Tetrabutyl titanate was dissolved in tertbutanol and heated to refluxing state by microwave irradiation. Then, lithium acetate was added into the mixed solution to produce LTO precursor. The precursor was hybridized with N,S-GQDs in ethanol. Followed by drying and thermal annealing at 500 °C in Ar/H 2 to obtain LTO/N,S-GQDs. The synthesis creates fully crystalline interconnected porous framework composed of nanoscale LTO crystals. The unique architecture achieves to maximize the high-rate performance and enhance the power density. More importantly, the introduction of N,S-GQDs don't almost influence on the electrolyte transport, but greatly improve the electron transfer and the storage lithium capacity. The LTO/N,S-GQDs anode exhibits remarkably enhanced electrochemical performance for lithium ion battery. The specific discharge capacity is 254.2 mAh g −1 at 0.1C and 126.5 mAh g −1 at 10C. The capacity remains 96.9% at least after 2000 cycles at 2C. The battery performance is significantly better than that of pure LTO electrode and LTO/graphene electrode.

Comparison of photoresponse of transistors based on graphene-quantum dot hybrids with layered and bulk heterojunctions

Phototransistors based on graphene–quantum dot hybrids have a high responsivity and gain. However, the influence of the type of heterojunction on the photoresponse of the transistors is still undetermined. A comparison was performed on field-effect phototransistors (FEpTs) with two types of heterojunctions: layered heterojunctions (LHs) and bulk heterojunctions (BHs). Through a comparative study, it was shown that BH–FEpTs had electron and hole mobilities (μE and μH) of 677 and 527 cm2 V−1 s−1 whereas LH–FEpTs had lower mobilities of μE = 314 cm2 V−1 s−1 and μH = 367 cm2 V−1 s−1. The large interfacial area in the BHs reduced the degree of channel order (α) by two orders of magnitude compared with the LHs. Although a higher mobility was achieved, an increase in the degree of channel disorder and the lack of an effective transfer mechanism limits the responsivity in BH–FEpTs. Therefore, LH–FEpTs are more appropriate candidates for near infrared phototransistors.

Gold nanoparticle integrated with nanostructured carbon and quantum dots: synthesis and optical properties

Multilayer graphene shell encapsulated gold nanoparticle—quantum dot hybrids were derived by combining wet-chemical, thermal, and covalent chemistry approaches. Uniformly patterned gold nanoparticles on a silicon substrate were obtained via gold film deposition in an electroless method followed by a thermal dewetting process. The resulting gold nanoparticles were further surface oxidized and utilized as catalysts for the chemical vapor deposition growth of multilayer graphene shell encapsulated on the gold nanoparticles (referred as “multilayer graphene shell encapsulated Au nanoparticle” or graphene nanoparticles (GNPs)). As a next step, the surface of GNPs was modified to result in carboxylic (−COOH) functionalities, which enabled carbodiimide-based covalent linking of amine-terminated CdS x Se1-x @ZnS quantum dots (QDs) on the GNP surface. The GNPs and GNP-QD heterostructures were characterized using scanning and transmission electron microscopy for size, morphology, spatial distribution, and crystal structure evaluation. In addition, UV-vis, fluorescence spectroscopy, and discrete dipole approximation (DDA) modeling were utilized for understanding the band gap energies, fluorescence quenching, and light-matter interactions of the derived hybrids/heterostructures.

The fluorescent interactions between amphiphilic chitosan derivatives and water-soluble quantum dots

The LCC–CdTe quantum dots (QDs) hybrid was fabricated by mixing the N-lauryl-N, O-carboxymethyl chitosan (LCC) micelle with water-soluble CdTe QDs in an aqueous solution via hydrophobic forces and the electronic attraction. The structures of LCC and LCC–CdTe QDs hybrid were determined by differential scanning calorimetry (DSC), Fourier transform infrared (FT-IR) spectroscopy and transmission electron microscopy (TEM). The results showed that the lauryl and carboxymethyl were successfully grafted to chitosan oligosaccharide (CSO), and a number of CdTe QDs were encapsulated by LCC micelle to form a core/shell structure. The tested results of the fluorescent characteristics of LCC, CdTe QDs and LCC–CdTe QDs hybrid showed that there were some obvious fluorescent interactions between LCC and CdTe QDs. Meanwhile, with the change in LCC space structure, the fluorescent interactions between LCC and QDs showed different fluorescent characteristics. The QDs fluorescent (FL) intensity increased first and then decreased to almost quenching, while LCC FL intensity decreased continually.

Generation of ultrashort extreme-ultraviolet pulses by enhanced plasmonic near-fields in metallic nanoparticles

We investigate high-order harmonic generation in a metallic-nanoparticle–quantum-dot hybrid system. Driven by a moderate electric field intensity Guassian-shape few-cycle pulse which is chirped by a hyperbolic-tangent-form carrier envelope phase, the hybrid system can generate extreme-ultraviolet super-continuum harmonics (13.3 eV maximal photon energy) and isolated ultrashort pulses ( FWHM). Numerical simulation results reveal that the plasmonic field enhancement could extend the cutoff order and continuum region in harmonic spectra. The extension effect by plasmonic enhancement could be well understood theoretically by the semi-classical two-level atom model.

Bulk- and layer-heterojunction phototransistors based on poly [2-methoxy-5-(2′-ethylhexyloxy-p-phenylenevinylene)] and PbS quantum dot hybrids

The responsivity (R) of a thin film photodetector is proportional to the product of its photo-induced carrier density (n) and mobility (μ). However, when choosing between layer heterojunction (LH) and bulk heterojunction (BH) field-effect phototransistors (FEpTs), it is still unclear which of the two device structures is more conducive to photodetection. A comparison study is performed on the two structures based on polymer and PbS quantum dot hybrids. Both devices exhibit ambipolar behavior, with μE ≈ μH = 3.7 cm2 V−1 s−1 for BH-FEpTs and μH = 36 cm2 V−1 s−1 and μE = 52 cm2 V−1 s−1 for LH-FEpTs. Because of the improvements in μ and the channel order degree (α), the responsivity of LH-FEpTs is as high as 101 A/W, which is as much as two orders of magnitude higher than that of BH-FEpTs (10−1A/W) under the same conditions. Although the large area of the BH improves both the exciton separation degree (β) and n in the BH-FEpT, the lack of an effective transport mechanism becomes the main constraint on high device responsivity. Therefore, LH-FEpTs are better candidates for use as photo detectors, and a “three-high” principle of high α, β, and μ is found to be required for high responsivity. At the request of the authors, this article is being retracted effective 23 February 2017.

Growth mechanism, optical and photocatalytic properties of ZnO nanorods@nanoflowers (quantum dots) hybrid nanostructures

ZnO nanorods@nanoflowers (quantum dots) hybrid nanostructures have been successfully fabricated by the spin coating method. The transmission electron micrograph (TEM) and field-emission scanning electron microscopy (FE-SEM) images showed that ZnO quantum dots (QDs) and ZnO nanoflowers (NFs) had been assembled on the surfaces of ZnO nanorods (NRs) or in the upper parts of the gaps between the rods. The morphology of ZnO nanoflowers was formed through a general route of zero-dimensional (0D)→2D→3D, controlling by sodium dodecyl sulfate (SDS) during the experiment. The blue-shifts of Raman and PL spectra were observed for the hybrid nanostructures at room temperature, which were explained by the quantum confinement effects. Compared with the photocatalytic activities of the samples, ZnO NRs@NFs showed enhanced photocatalytic performance for the degradation of RhB under UV radiation, which could be attributed to the special structural feature with an open and porous nanostructured surface layer and the surface defects. ZnO NRs@NFs would show a wider application for its particular porous morphology.

Electric-dipole-induced spin resonance in a lateral double quantum dot incorporating two single-domain nanomagnets

On-chip magnets can be used to implement relatively large local magnetic field gradients in nanoelectronic circuits. Such field gradients provide possibilities for all-electrical control of electron spin qubits where important coupling constants depend crucially on the detailed field distribution. We present a double quantum dot (QD) hybrid device laterally defined in a GaAs/AlGaAs heterostructure which incorporates two single-domain nanomagnets. They have appreciably different coercive fields which allows us to realize four distinct configurations of the local inhomogeneous field distribution. We perform dc transport spectroscopy in the Pauli-spin blockade regime as well as electric-dipole-induced spin resonance (EDSR) measurements to explore our hybrid nanodevice. Characterizing the two nanomagnets we find excellent agreement with numerical simulations. By comparing the EDSR measurements with a second double QD incorporating just one nanomagnet we reveal an important advantage of having one magnet per QD: It facilitates strong field gradients in each QD and allows us to control the electron spins individually for instance in an EDSR experiment. With just one single-domain nanomagnet and common QD geometries EDSR can likely be performed only in one QD.

Organic-Semiconductor Quantum Dot Hybrids: Controlling Photoinduced Energy and Electron Transfer

Self-assembled monolayes of photoactive compounds on semiconductor surfaces have been a subject of active research for a few decades already [1]. Apart from fundamental interest in electronic interactions at organic-semiconductor interfaces, the driving force for this activity is wide range of promising applications such as photo-voltaics, molecular senor, and nano-electronics. Although many semiconductor were tested in designing such organic-semiconductor hybrids, the properties of bulk semiconductors cannot be tuned easily and most of the research efforts were focused at the organic counterpart by synthesis of organic compounds with varying electronic properties, and providing different orientation and distance form the semiconductor surface [2]. Recent invention of semiconductor quantum dots (QDs) has increased tunability of organic-semiconductor hybrids drastically by allowing to very the energetic properties of the semiconductor counterpart [3]. The latter is controlled by the size of QDs which limits the spatial confinement of electrons in QDs and has two effects. Firstly, the energies of valence and conduction bands can be tuned in a rather wide range which affect the electron transfer at the organic-semiconductor interface. Secondly, the optical properties, absorption and emission of QDs, also depend on the size. Thus QDs can play an active role in light harvesting by organic-QD hybrids. The main objectives of this study is to gain information on energy and electron transfer in organic-QD hybrids when both parts can absorb light, and find bases to control the direction of electron transfer between organic and semiconductor counterparts. The QDs used in this study are CdSe core and core/shell QDs of varying size. Organic compounds are phthalocyanine, porphyrin and fullerene derivatives. The study was carried out using a range of spectroscopy techniques including steady state and ultra-fast time resolved methods. The steady state emission quenching and reduction of the emission lifetime of both QDs and organic compounds were used as primary tools to ensure hybrid formation and to evaluate the degree of electronic interactions. Femtosecond to nanosecond transient absorption spectroscopy [4] was used then for selected systems to identify the intermediate states formed and to obtain quantitative information on the reaction rate constants. In the visible part of the spectrum the responses were dominated by the QDs showing some bleaching and reshaping of the QD ground state absorption. However, in the red and near infrared (NIR) part of the spectrum the responses from organic chromophores had comparable or, in some cases, stronger contribution than that from the QDs. Therefore the NIR part of the spectrum was more informative for identification of the intermediate states and establishing the reaction mechanisms. The results show that the photoinduced electron transfer from CdSe QDs to fullerene C60derivative takes place for QDs of different size. The photoinduced electron transfer to the QDs was observed for QD-phthalocyanine hybrids, in which case only large QDs (with emission in the red part of the spectrum) operated as electron acceptors, whereas for hybrids with smaller QDs only energy transfer was observed. This can be rationalized considering the dependence of the energy of QD conduction band lower edge on the size. The larger QDs have lower energy and are better electron acceptor. This property of QDs can be effectively used to control photophyiscal reactions in QD-organic hybrids.

Soft chemistry based sponge-like indium tin oxide (ITO) — a prospective component of photoanode for solar cell application

Previously we reported the synthesis of novel organic-inorganic composite indium tin oxide (ITO) foam precursor leading to the formation of “sponge-like” ITO by burning away the organics. This newly made sponge-like ITO possesses relatively high electrical conductivity due to phonon confinement with reasonable pore structure and may have potential application as functional materials in semiconducting dye absorbing layer in dye-sensitized solar cell (DSSC) and also as the receptor of electrons injected from the quantum dots (QDs) of organic-inorganic hybrid QD based solar cell. This report is a short review of “sponge-like” ITO described as a lecture note on its future use as an alternative new prospective material for photoanode of solar cell in the domain of sustainable energy.

High-sensitive electrochemical sensor of Sudan I based on template-directed self-assembly of graphene-ZnSe quantum dots hybrid structure

In this work, a very sensitive electrochemical sensor for Sudan I was established based on nanohybrids of graphene-ZnSe quantum dots. Positively charged cetyltrimethyl-ammonium bromide (CTAB) functionalized graphene was used as a template-directed building block to self-assemble negatively charged ZnSe quantum dots (QDs). The hybrids of ZnSe-CTAB-graphene exhibited strong enrichment effect on Sudan I and gave superior electrocatalytic activity towards the oxidation of Sudan I. Applying differential pulse voltammetry (DPV), the peak current of Sudan I is proportional to its concentration in the range from 0.004 to 2.0μM and 2.0 to 15.0μM. This report is the first on applying graphene-ZnSe hybrids for electrochemical sensing.

Synergistic photocurrent addition in hybrid quantum dot: Bulk heterojunction solar cells

We investigate the effect of a thin PbS quantum dot (QD) layer on the performance of hybrid quantum-dot-organic solar cells (QD-OSCs). The PbS QD layer is able to function as a photosensitizing layer to improve short circuit current density (JSC) and power conversion efficiency (PCE) by exploiting solar flux in the near infrared region up to 1100nm. The increase in JSC is well represented by changes observed in the external quantum efficiency of devices with and without the PbS QD layer, including the region of the first exciton transition where only the PbS QD layer absorbs. Remarkably, enhanced performance was observed in QD-OSCs consisting of just a 13nm thick PbS QD layer and 150nm PTB7:PC71BM layer, exhibiting a JSC of 17.0mAcm−2, and PCE of 8.30% (8.58% for champion device) compared to reference devices without PbS QD which produced a JSC of 15.4mAcm−2 and PCE of 7.56%.

A study of polaritonic transparency in couplers made from excitonic materials

We have studied light matter interaction in quantum dot and exciton-polaritonic coupler hybrid systems. The coupler is made by embedding two slabs of an excitonic material (CdS) into a host excitonic material (ZnO). An ensemble of non-interacting quantum dots is doped in the coupler. The bound exciton polariton states are calculated in the coupler using the transfer matrix method in the presence of the coupling between the external light (photons) and excitons. These bound exciton-polaritons interact with the excitons present in the quantum dots and the coupler is acting as a reservoir. The Schrödinger equation method has been used to calculate the absorption coefficient in quantum dots. It is found that when the distance between two slabs (CdS) is greater than decay length of evanescent waves the absorption spectrum has two peaks and one minimum. The minimum corresponds to a transparent state in the system. However, when the distance between the slabs is smaller than the decay length of evanescent waves, the absorption spectra has three peaks and two transparent states. In other words, one transparent state can be switched to two transparent states when the distance between the two layers is modified. This could be achieved by applying stress and strain fields. It is also found that transparent states can be switched on and off by applying an external control laser field.

Hybrid type-I InAs/GaAs and type-II GaSb/GaAs quantum dot structure with enhanced photoluminescence

We investigate the photoluminescence (PL) properties of a hybrid type-I InAs/GaAs and type-II GaSb/GaAs quantum dot (QD) structure grown in a GaAs matrix by molecular beam epitaxy. This hybrid QD structure exhibits more intense PL with a broader spectral range, compared with control samples that contain only InAs or GaSb QDs. This enhanced PL performance is attributed to additional electron and hole injection from the type-I InAs QDs into the adjacent type-II GaSb QDs. We confirm this mechanism using time-resolved and power-dependent PL. These hybrid QD structures show potential for high efficiency QD solar cell applications.

Charge transfer properties in P3HT:graphene capped InAs/GaAs QDs hybrid heterostructure for photovoltaic application

In this work we have developed hybrid organic/inorganic nanostructure based on InAs/GaAs quantum dots (QDs) capped with P3HT:graphene organic nanocomposite using spin-coating method. InAs/GaAs QDs are grown by molecular beam epitaxy on n+-GaAs (001) substrate. The morphological properties are studied through atomic force microscopy (AFM) and scanning tunneling microscopy (STM). Raman spectroscopy analyzes show that the encapsulation of the InAs/GaAs QDs by P3HT:graphene nanocomposite keeps unchanged the indium concentration in the quantum dots due to the negligible strain effect of the organic cap layer on the QDs compared with the conventional GaAs cap layer. The quenching of the photoluminescence (PL) intensity of P3HT proves that an electrons transfer process from P3HT to graphene and from P3HT to InAs/GaAs QDs has occurred. We show that the proposed P3HT:graphene cap layer could replace the conventional GaAs cap layer for the elaboration of optoelectronic devices.

Hybrid quantum dot–superconducting systems: Josephson current and Kondo effect in the narrow-band limit

The case of a quantum dot connected to two superconducting leads is studied by using the narrow-band limit to describe the superconducting degrees of freedom. The model provides a simple theoretical framework, almost analytical, to analyze the interplay between the Kondo effect, superconductivity, and finite temperature. In the quantum dot Kondo regime, the model is completely characterized by the ratio $\ensuremath{\Delta}/J$, with $\ensuremath{\Delta}$ the superconducting gap and $J$ an effective antiferromagnetic exchange coupling between the dot and the leads. The model allows us to calculate, at any temperature $T$, the equilibrium Josephson current through the dot in a very straightforward way as a function of $\ensuremath{\Delta}/J$. The behavior of the current allows us to distinguish the four types of hybrid junctions: 0, ${0}^{\ensuremath{'}},{\ensuremath{\pi}}^{\ensuremath{'}}$, and $\ensuremath{\pi}$. The presence of the 0- and ${0}^{\ensuremath{'}}$-junction configurations are intrinsically linked to the Kondo effect in the quantum dot, while the $\ensuremath{\pi}$- and ${\ensuremath{\pi}}^{\ensuremath{'}}$-junction configurations are driven by the superconductivity in the leads. The Josephson critical current has a non-monotonic behavior with temperature, that may be used for the experimental characterization of the fundamental $0\ensuremath{-}\ensuremath{\pi}$ transition. The model allows us to obtain easily a phase diagram $\ensuremath{\Delta}/J$ vs temperature, from where we can obtain an overall picture on the stability of the different types of junctions. From the explicit analytical expressions for the ground-state, low-energy excitations, free energy, and Josephson current, it is easy to understand the physical nature of the main features of the critical current and the phase diagram. The results, obtained with a minimum of numerical effort, are in a good qualitative agreement with more demanding calculational approaches aimed to solve the full model.

Enhanced field emission of graphene–ZnO quantum dots hybrid structure

The cathode of graphene was prepared by the electrophoretic deposition (EPD) and ZnO quantum dots (QDs) were grown on the surface of graphene sheets by solution method to improve the field emission (FE) properties. The graphene/ZnO QDs hybrid emitters exhibited efficient field emission with lower turn-on field of 0.9V/μm, lower threshold field of 2.6V/μm, higher field enhancement factor of 3923 and more stable emission current stability than pristine graphene. The improved field emission performance was attributed to ZnO QDs, which introduce more defects, increase the number of emitting sites and decrease the work function. This investigation proposed that graphene/ZnO QDs composites are promising field cathodes in FE applications.

Copper nanoparticle/carbon quantum dots hybrid as green photocatalyst for high-efficiency oxidation of cyclohexane

Cu/CQDs hybrids as photocatalysts exhibit efficient photocatalytic activity in the oxidation of cyclohexane under visible light and mild conditions.