Growth shapes of supported Pd nanocrystals on SrTiO3(001)

Pd is deposited onto a reconstructed SrTiO3(001) substrate in an ultrahigh vacuum environment. Elevated substrate temperature during or following deposition causes epitaxial Pd nanocrystals to form. The nanocrystal shape and size distributions are analyzed by scanning tunneling microscopy. We find that depending on substrate reconstruction and substrate temperature during deposition three shapes of nanocrystal are obtained: truncated pyramids, huts, and hexagonal shaped disks. In our previous study [F. Silly and M. R. Castell, Phys. Rev. Lett. 94, 046103 (2005)] the energetics of the equilibrium nanocrystal shapes were analyzed. Here we report on the nonequilibrium growth shapes. We show that preferential growth for huts is along their (001) end facets. For hexagons growth proceeds by attachment to the side of the crystals. Truncated pyramids can grow preferentially along one of their (111) side facets resulting in an elongated shape. © 2005 The American Physical Society

Direct observation of highly anisotropic electronic and optical nature in indium telluride

Metal monochalcogenides (MX, M = Ga, In; X = S, Se, Te) offer a large variety of electronic properties depending on chemical composition, number of layers, and stacking order. InTe material has a one-dimensional chain structure, from which intriguing properties arise. Precise experimental determination of the electronic structure of InTe is needed for a better understanding of potential properties and device applications. In this study, by combining angle-resolved photoemission spectroscopy and density functional theory calculations, we demonstrate the stability of InTe in the tetragonal crystal structure, with a semiconducting character and an intrinsic p-type doping. The valence band maximum results in being located at the high symmetric M point with a high elliptical valley, manifesting a large effective mass close to the Fermi level. The longitudinal and transverse effective masses of the M valley are measured as 0.2 m(0) and 2 m(0), respectively. More specifically, we observe that the effective mass of the hole carriers is about ten times larger along the chain direction compared to the perpendicular one. Remarkably, the in-plane anisotropy of effective mass from the experiment and in theoretical calculations are in good agreement. These observations indicate a highly anisotropic character of the electronic band structure, making InTe of interest for electronic and thermoelectric applications

Dynamics in next-generation solar cells: time-resolved surface photovoltage measurements of quantum dots chemically linked to ZnO (101[combining macron]0)

The charge dynamics at the surface of the transparent conducting oxide and photoanode material ZnO are investigated in the presence and absence of light-harvesting colloidal quantum dots (QDs). The time-resolved change in surface potential upon photoexcitation has been measured in the m-plane ZnO (101[combining macron]0) using a laser pump-synchrotron X-ray probe methodology. By varying the oxygen annealing conditions, and hence the oxygen vacancy concentration of the sample, we find that dark carrier lifetimes at the ZnO surface vary from hundreds of μs to ms timescales, i.e. a persistent photoconductivity (PPC) is observed. The highly-controlled nature of our experiments under ultra-high vacuum (UHV), and the use of band-gap and sub-band-gap photoexcitation, allow us to demonstrate that defect states ca. 340 meV above the valence band edge are directly associated with the PPC, and that the PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for the PPC in ZnO. The effect of chemically linking two colloidal QD systems (type I PbS and type II CdS-ZnSe) to the surface has also been investigated. Upon deposition of the QDs onto the surface, the dark carrier lifetime and the surface photovoltage are reduced, suggesting a direct injection of charge carriers into the ZnO conduction band. The results are discussed in the context of the development of next-generation solar cells

Chemically-specific time-resolved surface photovoltage spectroscopy: Carrier dynamics at the interface of quantum dots attached to a metal oxide

All rights reserved.We describe a new experimental pump-probe methodology where a 2D delay-line detector enables fast (ns) monitoring of a narrow XPS spectrum in combination with a continuous pump laser. This has been developed at the TEMPO beamline at Synchrotron SOLEIL to enable the study of systems with intrinsically slow electron dynamics, and to complement faster measurements that use a fs laser as the pump. We demonstrate its use in a time-resolved study of the surface photovoltage of the m-plane ZnO (101¯0) surface which shows persistent photoconductivity, requiring monitoring periods on ms timescales and longer. We make measurements from this surface in the presence and absence of chemically-linked quantum dots (QDs), using type I PbS and type II CdSe/ZnSe (core/shell) QDs as examples. We monitor signals from both the ZnO substrate and the bound QDs during photoexcitation, yielding evidence for charge injection from the QDs into the ZnO. The chemical specificity of the technique allows us to observe differences in the extent to which the QD systems are influenced by the field of the surface depletion layer at the ZnO surface, which we attribute to differences in the band structure at the interface

Electronic band structure of two-dimensional WS2/Graphene van der Waals heterostructures

Combining single - layer two - dimensional semiconducting transition metal dichalcogenides (TMDs) with graphene layer in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these heterostructures . Her e , we report the electronic and structural properties of transferred single layer WS 2 on epitaxial graphene using micro - Raman spectroscopy, a ngle - resolv ed photoemission spectroscopy measurements (ARPES) and Density Functional Theory (DFT) calculations . The results show good electronic properties as well as well - defined band arising from the strong splitting of the single layer WS 2 valence band at K points , with a maximum splitting of 0.44 eV. By comparing our DFT results with local and hybrid functionals, we find the top valence band of the experimental heterostructure is close to the calculations for suspended single layer WS 2 . . Our results provide an i mportant reference for future studies of electronic properties of WS 2 and its applications in valleytronic devices

Photoluminescence properties of pyrolytic boron nitride.

We report on spectroscopic study of pyrolytichBN (pBN) by means of time- and energy-resolved pho-toluminescence methods. A high purity pBN samples(though low crystallinity) allow complementary informa-tion about excited states involved into the luminescenceprocess. We affirm our recent conclusions about theimpurity-related nature of most of fluorescence bands inmicrocrystalline hBN. In addition, a broad band centred at3.7 eV previously not considered because of its superpo-sition with an intense structured impurity emission isattributed to the radiative recombination of deep DAPs