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ABSTRACT: Using an electrochemically gated transistor, we achieved controlled and reversible doping of poly(p-phenylene vinylene) in a large concentration range. Our data open a wide energy-window view on the density of states (DOS) and show, for the first time, that the core of the DOS function is Gaussian, while the low-energy tail has a more complex structure. The hole mobility increases by more than 4 orders of magnitude when the electrochemical potential is scanned through the DOS.
Physical Review Letters 11/2004; 93(16):166601. · 7.37 Impact Factor
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Physical Review Letters 10/2003; 91(16):169704. · 7.37 Impact Factor
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ABSTRACT: Electron transport in an assembly of ZnO quantum dots has been studied using an electrochemically gated transistor. The electron mobility shows a stepwise increase as a function of the electron occupation per quantum dot. When the occupation number is below two, transport occurs by tunneling between the S orbitals. Transport becomes 3 times faster when the occupation number is between two and eight; tunneling now occurs between the P orbitals. Electron transport is thus critically determined by the quantum properties of the building blocks.
Physical Review Letters 08/2002; 89(3):036801. · 7.37 Impact Factor
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ABSTRACT: Colloidal CdSe quantum dots were chemisorbed on a gold electrode using a variety of self-assembled monolayers (SAMs) consisting of dithiols and rigid disulfides. After absorption of a photon with an energy larger than the band gap, a long-lived excited state is formed in the quantum dot; this state can decay by electron tunneling via the gold. The rate of photoinduced tunneling was measured directly by intensity-modulated photocurrent spectroscopy (IMPS), and its distance dependence was studied using rigid SAMs separating the Q-CdSe and Au. The tunneling rate was found to depend exponentially on the distance, with a decay length of 2 Å.
07/2000;
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ABSTRACT: The electrical properties of InP/Au dry contacts and the influence of electrodeposited gold on the electrochemical properties of InP electrodes were studied. From current and impedance measurements on InP/Au dry contacts it is concluded that the chemical composition of an interfacial layer strongly influences the electrical characteristics. Furthermore, interface states are shown to play an important role. The interfacial layer and the interface states also determine to a large extent the electrochemical properties of the gold‐plated electrodes.
Journal of Applied Physics 10/1993; · 2.17 Impact Factor
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ABSTRACT: Reduction of S 2 O 8 <sup>2-</sup> ions at the interface between an n‐type porous Si electrode and an aqueous solution gives rise to electroluminescence showing evidence of quantization effects. A broad emission band with a maximum at 670 nm is observed similar to the photoluminescence spectrum of the same layers. The results suggest that holes are ‘‘injected’’ into the valence band of the porous semiconductor from an intermediate of the reduction reaction, the SO 4 <sup>-∙</sup> radical ion. The resulting electron‐hole recombination is responsible for the visible light emission.
Applied Physics Letters 08/1992; · 3.84 Impact Factor
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ABSTRACT: We have studied the storage and long-range transport of electrons in a porous assembly of weakly coupled ZnO quantum dots permeated with an aqueous and a propylene carbonate electrolyte solution. The number of electrons per ZnO quantum dot is controlled by the electrochemical potential of the assembly; the charge of the electrons is compensated by ions present in the pores. We show with optical and electrical measurements that the injected electrons occupy the S, P, and D type conduction electron levels of the quantum dots; electron storage in surface states is not important. With this method of three-dimensional charge compensation, up to ten electrons per quantum-dot can be stored if the assembly is permeated with an aqueous electrolyte. The screening of the electron charge is less effective in the case of an assembly permeated with a propylene carbonate electrolyte solution. Longrange electron transport is studied with a transistor set-up. In the case of ZnO assemblies permeated with an aqueous electrolyte, two quantum regimes are observed corresponding to multiple tunnelling between the S orbitals (at a low occupation) and P orbitals (at a higher occupation). In a ZnO quantum-dot assembly permeated with a propylene carbonate electrolyte solution, there is a strong overlap between these two regimes.
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ABSTRACT: The temperature dependence of electron transport was studied in an assembly of ZnO quantum dots. The number of electrons per quantum dot was controlled by the electrochemical potential. For assemblies permeated with organic electrolyte solutions, the electron conductance measured in the linear response regime shows an activation energy of 80–120 meV. Our analysis indicates that this is due to Coulomb blockade of electron transport.
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ABSTRACT: Electron-conducting quantum-dot solids can be prepared on the basis of assemblies of colloidal insulating nanocrystals if electrons can be injected in the delocalized conduction orbitals. We discuss the energetics of electron injection in such an artificial solid consisting of weakly coupled quantum dots. We show that quantum confinement and electron-electron repulsion determine the charging characteristics. The electron-electron repulsion energy can be screened by three-dimensional charge compensation from trapped holes or positive inert ions inserted in the assembly. We present experimental results on the electron storage and long-range transport in assemblies of ZnO nanocrystals in which the electron charge is compensated by positive ions. The electron-electron repulsion energy in assemblies permeated with an aqueous electrolyte solution is strongly screened. In contrast, the repulsion energy is about 100 meV in aprotic solvents; the repulsion energy strongly influences electron storage and the characteristics of long-range electron transport.
Faraday Discussions. 125:55-62.
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ABSTRACT: The frequency dependence of the impedance of polarizable semiconductor/metal and semiconductor/electrolyte solution interfaces is reconsidered. No frequency dispersion of the polarization capacitance is found with n-GaAs/Au contacts whereas the capacitance of n-GaAs/electrolyte solution interfaces show considerable dispersion. The frequency dispersion depends on the microroughness of the electrode surface and on the specific conductivity of the electrolyte solution. As for polarizable metal electrodes, the relationship between the capacitance and the frequency is closely related to the relationship between the capacitance and the specific conductivity of the electrolyte solution. It is concluded that the main origin of frequency dispersion should not be sought in the solid but is related to the development of the electric double layer at the electrolyte side of the interface. A model is presented to account for these results.
Journal of Electroanalytical Chemistry and Interfacial Electrochemistry.
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ABSTRACT: Electrochemical polarization of a crystalline, polymeric or nanoporous system or a single molecule may change the density of charge carriers in a controlled way, and hence the optical and electrical properties. If the system has two contacts, its electronic conductivity can be measured in situ as a function of the charge carrier density that is varied by the electrochemical potential. This is called electrochemical gating. Such investigations can reveal the nature of the charge carriers (mobile or localized) and the mechanism of electronic conduction. Here, we present a brief review of a number of systems including inorganic crystals, polymers, nanoporous quantum-dot solids, and single molecules for which electrochemical gating was used successfully in the study of the electronic properties.
Electrochimica Acta.
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ABSTRACT: The visible luminescence of porous silicon electrodes is reported. Both n-type and p-type material show similar characteristics. Three potential ranges can be distinguished. At open-circuit potential in the presence of certain strong oxidizing agents, e.g. Ce4+ and MnO-4, emission due to chemiluminescence (CL) occurs. At negative potential in the range of H2 evolution, hole injection by a strong oxidizing agent, e.g. H2O2 and S2O2-8, results in electroluminescence (EL). The intensities of CL and EL depend on the applied potential. When porous silicon is anodically oxidized, emission also occurs; this is probably due to CL. Because of the close resemblance of the luminescence of porous silicon to the photo- and chemiluminescence of siloxene, the emission from porous silicon is ascribed to the presence of siloxene or siloxene-like groups. The porous layer can be described as a discrete semiconductor with a large band-gap.
Applied Surface Science.
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ABSTRACT: The influence of electrodeposited gold on the current—potential and impedance characteristics of n-GaAs electrodes was studied in various electrolyte solutions. The potential of the gold layer on n-GaAs was measured as a function of the bias applied to the electrode. It is concluded that the same processes occur as previously proposed for gold-plated p-GaAs electrodes. A model involving gold- related surface states which interact with both the valence band and the electrolyte solution was used to explain the results for p-GaAs. For n-GaAs, an additional process is found to play an important role: recombination of thermally produced holes with conduction band electrons.
Electrochimica Acta.
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ABSTRACT: The influence of electrodeposition of gold on the electrochemical properties of p-type GaAs has been studied. From current—potential and impedance measurements performed in solutions containing various redox-couples it was concluded that gold-related surface states play a central role in the interaction between semiconductor, metal and electrolyte solution. A kinetic model is presented which comprises three processes: (i) excitation of valence band electrons to the surface states, (ii) recombination of these electrons with holes in the valence band and (iii) transfer of electrons from the surface states to an electron acceptor in solution.
Electrochimica Acta.
