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Accumulation Layer (accumulation + layer)
Selected AbstractsObservation of a 2D Electron Gas and the Tuning of the Electrical Conductance of ZnO Nanowires by Controllable Surface Band-BendingADVANCED FUNCTIONAL MATERIALS, Issue 15 2009Youfan Hu Abstract Direct experimental evidence for the existence of a 2D electron gas in devices based on ZnO nanowires (NWs) is presented. A two-channel core/shell model is proposed for the interpretation of the temperature-dependent current,voltage (I,V) characteristics of the ZnO NW, where a mixed metallic,semiconducting behavior is observed. The experimental results are quantitatively analyzed using a weak-localization theory, and suggest that the NW is composed of a "bulk" semiconducting core with a metallic surface accumulation layer, which is basically a 2D electron gas in which the electron,phonon inelastic scattering is much weaker than the electron,electron inelastic scattering. A series of I,V measurements on a single NW device are carried out by alternating the atmosphere (vacuum, H2, vacuum, O2), and a reversible change in the conductance from metallic to semiconducting is achieved, indicating the surface accumulation layer is likely hydroxide-induced. Such results strongly support the two-channel model and demonstrate the controllable tuning of the ZnO NW electrical behavior via surface band-bending. [source] Inferring nocturnal surface fluxes from vertical profiles of scalars in an Amazon pastureGLOBAL CHANGE BIOLOGY, Issue 5 2004Otávio C. Acevedo Abstract Ecosystem carbon budgets depend on there being good representative surface flux observations for all land use types during the entire diurnal cycle. In calm conditions that often occur at night, especially in areas of small roughness (such as pastures), ecosystem respiration rate is poorly measured using the eddy covariance (EC) technique. Nocturnal vertical profiles of temperature, humidity and winds were observed using tethered balloon soundings in a pasture in the eastern Amazon during two campaigns in 2001. The site is characterized by very weak winds at night, so that there is insufficient turbulence for the EC technique to determine fluxes accurately. To compensate, the time evolution of the profiles is used to determine surface fluxes at early morning and these are compared with those observed by EC at a nearby micrometeorological tower. The nocturnal boundary layer thickness h is determined as the height to which the surface fluxes must converge so that energy budget closure is achieved. The estimated values range from 30 m, around 22:00 hours LST, to more than 100 m just before dawn. These are in good agreement with the observed thickness of a frequently observed fog layer during the middle of the night. During the early portion of the night, when the accumulation layer is shallow, there is appreciable decrease of dCO2/dt with height. On calm nights, CO2 accumulation rate is larger near the surface than at higher levels. On windier nights, this accumulation rate is vertically uniform. Hence, extrapolation of tower profiles for estimating fluxes must be done carefully. Although uncertainties remain large, an alternate approach to the EC method is described for measuring nighttime surface CO2 fluxes under stable atmospheric conditions. [source] Surface-Transfer Doping of Organic Semiconductors Using Functionalized Self-Assembled Monolayers,ADVANCED FUNCTIONAL MATERIALS, Issue 8 2007W. Chen Abstract Controlling charge doping in organic semiconductors represents one of the key challenges in organic electronics that needs to be solved in order to optimize charge transport in organic devices. Charge transfer or charge separation at the molecule/substrate interface can be used to dope the semiconductor (substrate) surface or the active molecular layers close to the interface, and this process is referred to as surface-transfer doping. By modifying the Au(111) substrate with self-assembled monolayers (SAMs) of aromatic thiols with strong electron-withdrawing trifluoromethyl (CF3) functional groups, significant electron transfer from the active organic layers (copper(II) phthalocyanine; CuPc) to the underlying CF3 -SAM near the interface is clearly observed by synchrotron photoemission spectroscopy. The electron transfer at the CuPc/CF3 -SAM interface leads to an electron accumulation layer in CF3 -SAM and a depletion layer in CuPc, thereby achieving p-type doping of the CuPc layers close to the interface. In contrast, methyl (CH3)-terminated SAMs do not display significant electron transfer behavior at the CuPc/CH3 -SAM interface, suggesting that these effects can be generalized to other organic-SAM interfaces. Angular-dependent near-edge X-ray absorption fine structure (NEXAFS) measurements reveal that CuPc molecules adopt a standing-up configuration on both SAMs, suggesting that interface charge transfer has a negligible effect on the molecular orientation of CuPc on various SAMs. [source] Polaron Localization at Interfaces in High-Mobility Microcrystalline Conjugated PolymersADVANCED MATERIALS, Issue 37 2009N. Zhao The charge-induced optical absorptions of two-dimensional polarons in semicrystalline, high-mobility conjugated polymers are investigated as a function of distance from an interface with polymer gate dielectrics of different dielectric constants. For high- k dielectrics, polarons in the accumulation layer at the interface are found to be more localized than those in the bulk. [source] | ||||||||||