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ABSTRACT: We report the use of scanning tunneling spectroscopy (STS) to investigate one-dimensional quantum confinement effects in lead sulfide (PbS) thin films. Specifically, quantum confinement effects on the band gap of PbS quantum wells were explored by controlling the PbS film thickness and potential barrier height. PbS quantum well structures with a thickness range of 1-20 nm were fabricated by atomic layer deposition (ALD). Two barrier materials were selected based on barrier height: aluminum oxide as a high barrier material and zinc oxide as a low barrier material. Band gap measurements were carried out by STS, and an effective mass theory was developed to compare the experimental results. Our results show that the band gap of PbS thin films increased as the film thickness decreased, and the barrier height increased from 0.45 to 2.19 eV.
Nanotechnology 12/2010; 21(48):485402. · 3.98 Impact Factor
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ABSTRACT: Al-doped ZnO (AZO) films were deposited by atomic layer deposition (ALD) on borosilicate glass and sapphire(0001) substrates. The Al composition of the films was varied from 1% to 4% by controlling the ratio of Zn:Al pulses. Film resistivity was measured as a function of Al content and the substrate temperature used for ALD deposition. X-ray diffraction (XRD) was performed on the films, showing a reduction in lattice parameter, as a function of Al concentration, indicating that Al3+ ions occupy substitutional sites in the ZnO lattice. The resistivity of films deposited on sapphire substrates (7.7 × 10−4 Ω cm) was lower than that on glass (3.0 × 10−3 Ω cm), because of the formation of textured grains with the c-axis aligned with respect to the sapphire surface, as confirmed by XRD. The surface morphology of the films on glass and sapphire was compared using scanning tunneling microscopy (STM) and scanning electron microscopy (SEM), which showed similar grain sizes on each substrate, suggesting that the difference in conductivity was due to grain orientation rather than microstructural differences. Optical transparency was measured to be >80% for wavelengths of 370−1600 nm.
07/2010;
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ABSTRACT: Area-selective atomic layer deposition (ALD) of lead sulfide (PbS) was achieved on octadecyltrichlorosilane (ODTS)-patterned silicon substrates. We investigated the capability of ODTS self-assembled monolayers (SAMs) to deactivate the ALD PbS surface reactions as a function of dipping time in ODTS solution. The reaction mechanism was investigated using density functional theory (DFT), which showed that the initial ALD half-reaction is energetically unfavorable on a methyl-terminated SAM surface. Conventional photolithography was used to create oxide patterns on which ODTS SAMs were selectively grown. Consequently, PbS thin films were grown selectively only where ODTS was not present, whereas deposition was blocked in regions where ODTS was grown. The resulting fabricated patterns were characterized by scanning electron microscopy and Auger electron spectroscopy, which demonstrated that ALD PbS was well confined to defined patterns with high selectivity by ODTS SAMs. In addition, AFM lithography was employed to create nanoscale PbS patterns. Our results show that this method can be applied to various device-fabrication processes, presenting new opportunities for various nanofabrication schemes and manifesting the benefits of self-assembly.
Langmuir 05/2010; 26(9):6845-52. · 4.19 Impact Factor
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ABSTRACT: Local accumulation and dissipation of charges on the surface of oxide ion conductors resulting from electric potentials were observed with conductive atomic force microscopy (AFM). After a negative bias was applied at the tip, a sequence of surface potential maps appeared compatible with electron injection onto the electrolyte surface. Applying a positive bias, in contrast, generated a positive surface charge adjacent to the tip contact area. This observation is consistent with the formation of oxide ion vacancies on the oxide surface. In addition, oxide ion conductivity at a low temperature range (100-200 degrees C) was obtained, and the activation energy for diffusion in gadolinia-doped ceria (GDC) was calculated as approximately 0.56 eV, implying that the majority of oxide ion vacancies diffuse on the surface rather than inside the bulk of the electrolyte.
Nanotechnology 11/2009; 20(44):445706. · 3.98 Impact Factor
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ABSTRACT: Lead sulfide (PbS) thin films were deposited by atomic layer deposition (ALD) for the fabrication of quantum well structures. A linear growth rate of 0.9 Å/cycle and pulse saturation behavior characteristic of ALD were observed. The stoichiometry of the films was confirmed using X-ray photoelectron spectroscopy (XPS) with no chemical contamination. The polycrystalline film morphology was observed with grain sizes ranging from 30 to 150 nm. Size quantization effects are shown on the bandgap by fabricating PbS quantum wells with a sub-10 nm thickness. Bandgap values were measured by tunneling spectroscopy (TS) using scanning tunneling microscopy (STM) and are matched to an effective mass model. The bandgap of the films was changed from 0.4 to 2.75 eV by varying only the number of ALD cycles.
08/2009;
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ABSTRACT: We report the use of the scanning tunneling spectroscopy (STS) to investigate 1-dimensional quantum confinement effects in lead sulfide (PbS) thin films. The band gap was varied by control of the PbS film thickness and barrier materials. PbS quantum well structures with a thickness range of 1-20 nm were prepared by atomic layer deposition (ALD) due to its unique characteristics: precise thickness control with sub-nm resolution, pinhole-free films, and conformal coating. Two barrier materials were selected based on their barrier height: silicon dioxide as a high barrier material and zinc sulfide as a low barrier material. PbS quantum wells embedded in different barrier materials were characterized with STS to measure band gap variations. Experimental results showed that the band gap of PbS thin films increased as film thickness decreased and barrier height increased. The experimental results showed good agreement with an effective mass model.
Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE; 07/2009
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ABSTRACT: Quantum confinements such as quantum wells, wires, and dots posses several advantages for next-generation solar cells. In this study, we present results on quantum confinement in PbS-ZnS quantum wells deposited by Atomic Layer Deposition (ALD). Materials selection criteria are presented with a focus on the properties of the well and barrier material. PbS quantum wells embedded in thin ZnS barrier layers are shown to demonstrate quantum confinement effects through scanning tunneling microscopy (STM). The band gap of the PbS films has been varied from 0.4-1.0 eV by varying the number of ALD cycles. The bandgap variation with film thickness is well matched to results predicted by effective-mass theory.
Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE; 07/2009
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ABSTRACT: Electrostatic force microscopy (EFM) is a special design of non-contact atomic force microscopy used for detecting electrostatic interactions between the probe tip and the sample. Its resolution is limited by the finite probe size and the long-range characteristics of electrostatic forces. Therefore, quantitative analysis is crucial to understanding the relationship between the actual local surface potential distribution and the quantities obtained from EFM measurements. To study EFM measurements on bimetallic samples with surface potential inhomogeneities as a special case, we have simulated such measurements using the boundary element method and calculated the force component and force gradient component that would be measured by amplitude modulation (AM) EFM and frequency modulation (FM) EFM, respectively. Such analyses have been performed for inhomogeneities of various shapes and sizes, for different tip-sample separations and tip geometries, for different applied voltages, and for different media (e.g., vacuum or water) in which the experiment is performed. For a sample with a surface potential discontinuity, the FM-EFM resolution expression agrees with the literature; however, the simulation for AM-EFM suggests the existence of an optimal tip radius of curvature in terms of resolution. On the other hand, for samples with strip- and disk-shaped surface potential inhomogeneities, we have obtained quantitative expressions for the detectability size requirements as a function of experimental conditions for both AM- and FM-EFMs, which suggest that a larger tip radius of curvature is moderately favored for detecting the presence of such inhomogeneities.
Nanotechnology 01/2008; 19(3):035710. · 3.98 Impact Factor
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ABSTRACT: Due to the finite scanning probe microscopy (SPM) tip radius and the resulting geometric convolution between the tip and the sample surface, nano-resolution surface potential (SP) or electric force measurement (EFM) cannot be free from topographic artefacts. For conventional Kelvin probe microscopy (KPM), only the first harmonic component of the tip oscillation signal (either oscillation amplitude for amplitude-modulated AM-KPM or frequency shift for frequency-modulated FM-KPM) induced by the applied ac voltage is typically used. However, the first harmonic signal depends not only on tip–sample potential difference, but also on the capacitance gradient (AM-KPM) or the second-order gradient (FM-KPM), the main cause of topographical artefacts. Since the second-order harmonic component is proportional only to the capacitance gradient or second-order gradient, we are able to extract true potential difference signals, free of geometric artefacts, by dividing the first- and second-order harmonics. Surface potential mapping on an equipotential surface verifies that this alternative method significantly reduces the magnitude of topological artefacts. In addition, adoption of the proposed imaging method reduces the dependence of the measured potential on the tip–sample separation by more than an order of magnitude.
Nanotechnology 06/2006; 17(15):3728. · 3.98 Impact Factor
