Thermal annealing behaviour of Pt on n-GaN Schottky contacts

The structural evolution and temperature dependence of the Schottky barrier heights of Pt contacts on n-GaN epilayer at various annealing temperatures were investigated extensively by Rutherford backscattering spectrometry, x-ray diffraction measurements, Auger electron spectroscopy, scanning electron microscopy and current-voltage measurements. The temperature dependence of the Schottky barrier heights may be attributed to changes of surface morphology of Pt films on the surface and variation of nonstoichiometric defects at the interface vicinity. Experimental results indicated the degradation of Pt contacts on n-GaN above 600 degreesC.

Characterization of deep levels in Pt-GaN Schottky diodes deposited on intermediate-temperature buffer layers

Gallium nitride (GaN)-based Schottky junctions were fabricated by RF-plasma-assisted molecular beam epitaxy (MBE). The GaN epitaxial layers were deposited on novel double buffer layers that consist of a conventional low-temperature buffer layer (LTBL) grown at 500 degreesC and an intermediate-temperature buffer layer (ITBL) deposited at 690 degreesC. Low-frequency excess noise and deep level transient Fourier spectroscopy (DLTFS) were measured from the devices. The results demonstrate a significant reduction in the density of deep levels in the devices fabricated with the GaN films grown with an ITBL. Compared to the control sample, which was grown with just a conventional LTBL, a three-order-of-magnitude reduction in the deep levels 0.4 eV below the conduction band minimum (Ec) is observed in the bulk of the thin films using DLTFS measurements.

Hydrogen sensors based on Pt-AlGaN/GaN back-to-back Schottky diode

In this paper, platinum (Pt) with a thickness of 45 nm was sputtered on the surface of AlGaN/GaN heterostructure to form the Schottky contact and the back-to-back Schottky diodes were characterized for H-2 sensing at room temperature. Both the forward and reverse current of the devices increased with exposure to H-2 gas, which was attributed to Schottky barrier height reduction caused by hydrogen absorption in the catalytic metals. A shift of 0.7 V at 297 K was obtained at a fixed forward current of 0.1 mA after switching from N-2 to 40% H-2 in N-2. The sensor's responses under different concentrations from 2500 ppm H-2 to 40% H-2 in N-2 at 297 K were investigated. Time response of the sensor at a fixed bias of 1 V was given. Finally, the decrease of the Schottky barrier height and the sensitivity of the sensor were calculated. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Hydrogen sensors based on Pt-AlGN/AIN/GaN Schottky diode - art. no. 68291R

Pt/AlGaN/AlN/GaN Schottky diodes have been fabricated and characterized for H-2 sensing. Platinum (Pt) with a thickness of 20nm was evaporated on the sample to form the Schottky contact. The ohmic contact, formed by evaporated Ti/Al/Ni/Au metals, was subsequently annealed by a rapid thermal treatment at 860 degrees C for 30 s in N-2 ambience. Both the forward and reverse current of the device increased greatly when exposed to H-2 gas. The sensor's responses under different hydrogen concentrations from 500ppm to 10% H-2 in N-2 at 300K were investigated. A shift of 0.45V at 297K is obtained at a fixed forward current for switching from N-2 to 10% H-2 in N-2. Time response of the sensor at a fixed bias of 0.5 V was also measured. The turn-on response of the device was rapid, while the recovery of the sensor at N-2 atmosphere was rather slow. But it recovered quickly when the device was exposed to the air. The decrease in the barrier height of the diode was calculated to be about 160meV upon introduction of 10% H-2 into the ambient. The sensitivity of the sensor is also calculated. Some thermodynamics analyses have been done according to the Langmuir isotherm equation.

CFCs和PT方程

CFCs问题引来了广泛的研究.本文概述了CFCs问题.对于状态方程，讨论了1982年发表的PT方程。由此进一步比较讨论了RKS 和PR方程，另外总结了立方型方程的通式.这些对CFCs新工质的模拟计算都具有现实价值.

Effect of Ru addition to CO/SiO2/HZSM-5 catalysts on Fischer-Tropsch synthesis of gasoline-range hydrocarbons

Microporous HZSM-5 zeolite and mesoporous SiO2 supported Ru-Co catalysts of various Ru adding amounts were prepared and evaluated for Fischer-Tropsch synthesis (FTS) of gasoline-range hydrocarbons (C-5-C-12). The tailor-made Ru-Co/SiO2/HZSM-5 catalysts possessed both micro- and mesopores, which accelerated hydrocracking/hydroisomerization of long-chain products and provided quick mass transfer channels respectively during FTS. In the same time. Ru increased Cor reduction degree by hydrogen spillover, thus CO conversion of 62.8% and gasoline-range hydrocarbon selectivity of 47%, including more than 14% isoparaffins, were achieved simultaneously when Ru content was optimized at 1 wt% in Ru-Co/SiO2/HZSM-5 catalyst.

全固态电流型氧传感器及Pt-Fe复合催化剂的研究

控制电位电解型即电流型气体传感器由于具有检测气体种类多、浓度范围宽、体积小、价格低、测量精度高、可用于现场直接检测等优点，在环境监测与安全生产等领域中得到了广泛应用。本文论述了纳米级铂、碳纳米管负载铂及铂铁催化剂的合成方法，并对其进行了物理化学表征和电化学研究，主要结果如下： 采用柠檬酸钠还原的方法，合成了纳米级铂催化剂。TEM和XRD测试表明，该球形多晶铂的粒径为2－5nm。用此种铂催化剂制备了离子交换膜电极复合体，并组装了全固态电流型氧传感器；在对低浓度氧气进行测试时，具有高的灵敏度、较短的响应时间、较低的底电流和噪声，且响应信号与氧气的浓度呈良好的线性关系。 1．利用空气氧化和硝酸相结合对多壁碳纳米管进行了纯化，并利用1:1的H2SO4-HNO3混酸使其表面羧基化，TEM和CV测试表明多壁碳纳米管达到了纯化和表面官能团化的目的。 2．利用喷雾冷却法制备了多壁碳纳米管负载的铂及铂铁合金纳米级催化剂复合体，并利用TEM、EDS、XRD、ICP等进行了表征；在循环伏安扫描中，铂铁合金催化剂呈现较高的比表面积。将多壁碳纳米管负载的铂及铂铁合金纳米级催化剂应用于C1有机小分子的电化学氧化研究中发现，铂铁合金催化剂具有较高的催化活性.

贵金属（Au, Ag, Pt, Pd）纳米材料的可控合成及性能研究

功能化的贵金属纳米材料的设计和可控制备在材料科学研究领域引起了人们广泛的关注。贵金属纳米材料的光学、电学、磁学和催化等物理和化学性质不但与其大小有关，而且还与其形貌息息相关。因此寻求简单而有效的低温溶液合成途径以实现对贵金属纳米材料的尺寸和形貌控制尤为重要。本论文的主要研究内容可以归纳如下： (1)在水溶液中利用种子生长方法分别制备了核壳Au-Pd/Pt三金属复合纳米粒子和三层的核壳AuAg复合纳米粒子。这些纳米粒子的尺寸和组成可以通过改变金种子的加入量来加以调控。 (2)通过种子生长和取代沉积相结合的方法制备了具有金核铂/银双金属壳的铃铛状纳米粒子。通过扫描电子显微镜、透射电子显微镜和X-射线光电子能谱对所得纳米粒子的尺寸、结构和组成进行了表征。 (3)以二肽甘氨酰甘氨酸作为模板合成了具有[111]取向的单晶银纳米片。通过改变实验条件探讨了片状银纳米结构的形成机理。片状银纳米结构的产率可达到80%，反应物之间的摩尔比对产物的尺寸和形貌有至关重要的作用。 (4)将K3[Fe(CN)6]和Na2S2O3的混合溶液进行水热处理，得到了具有立方体形貌的FeIIIFeIII(CN)6（柏林绿）微晶。实验结果显示K3[Fe(CN)6]和Na2S2O3的摩尔比及其浓度对所得产物的尺寸、形貌和组成有决定性的作用。 (5)在室温下通过混合3, 3', 5, 5'-四甲基联苯胺和氯铂酸，成功合成了有机-无机杂化的纳米纤维。纳米纤维的尺寸和形状可以通过改变反应物的比和浓度加以控制。基于不同的实验结果，提出了纳米纤维的可能形成机理。

Cr(III)盐在Pt电极和PbO_2电极上阳极氧化为络酸盐的机理的研究

关于Cr(III)阳极氧化为Cr(VI)的过程，在工业应用方面已有许多工作，但对机理尚不清楚。本文对Pt电极和PbO_2电极上的这一过程进行了动力学的研究，首次得到了Cr(VI)在这两种电极上阳极形成的真实动力学数据，并提出了与实验基本相符的反应机理。用“分解极化曲线法”和稳态极化曲线的测量，得到了[H_2SO_4]和[SO_4~=]恒定的不同浓度Cr_2(SO_4)_3溶液中Cr(VI)在光滑Pt电极上阳极形成的真实动力学数据，可用如下方程式来表达：在较低电位下φ = a + 0.25 log i_2 - 0.23 log [Cr(III)]在较高电位下φ = a' + 0.48 log i_2 - 0.44 log [Cr(III)]同时得到了氧的阳极发生的动力学数据，Tafel线性区的斜率接近于2.303 RT/ΔF (Δ ≈ 0.5)。动力学方程式的推导与实验的比较表明，在光滑Pt电极上Cr(VI)是由Cr(II)通过“活性氧”的氧化形成的，提出了如下反应机理：H_2O → (OH)_(ad) + H~+ + e~- 2(OH)_(ad) → (O)_(ad) + H_2O 2(O)_(ad) →O_2 (OH)_(ad) + [Cr(H_2O)_6]~(3+) → (CrO_2)_(ad) + 3H~+ + 5H_2O (CrO_2)_(ad) + H_2O → (CrO_3~-)_(ad) + 2H~+ + e~- (CrO_3~-)_(ad) + H_2O → HCrO_4~- + H~+ + e~- 2HCrO_4~- <-> Cr_2O_7~(2-) + H_2O 在[H_2SO_4]和[SO_4~(2-)]恒定的不同浓度Cr_2(SO_4)_3溶液中，测得了Cr(VI)在Δ-PbO_2电极上阳极形成的动力学数据：在较低电位下φ = a + 0.28 log i_2 - 0.30 log [Cr(III)]在较高电位下φ = a' + 0.55 log i_2 - 0.51 log [Cr(III)]氧的极化曲线的Tafel线性区斜率也为2.303RT/ΔF (Δ approx= 0.5)。PbO_2电极和Pt电极上分解极化曲线的比较表明，Cr(VI)在前一电极上阳极形成的过电位远低于在后一电极上，这可能是两电极上电流效率显著差别的原因。测得了PbO_2电极上不同过电位下电极反应的有效活化能，其数值均在10 Kcal mol~(-1)以上，且随着极化的增大而减小，据此在动力学处理中可以忽略扩散的作用，交流阻抗的研究进一步证实了这一点。溶液pH的增大或[H_2SO_4]的减小会降低Cr(VI)阳极形成的过电位，使反应加速。在[H_2SO_4]和[SO_4~(2-)]恒定的不同浓度Cr_2(SO_4)_3溶液中，测得了在不同电位极化下PbO_2电极的阻抗频谱。在较高电位下阻抗谱呈现明显的两个半圆，表明电极过程包括了中间吸附物的形成。求得的吸附电容Cad比双层电容Cd大1-2个数量级；Cd比通常光滑电极表面的Cd大得多，这可能与SO_4~(2-)，HCrO_4~-, Cr_2O_7~-以及[_((SO_4))~((H_2O)_2) Cr_((SO_4))~((OH)_2) Cr_((SO_4))~((OH_2)_2)]~(2-)等阳离子的特性吸附有关。从动力学的推导与实验的比较得出，在PbO_2电极上Cr(VI)也按“活性氧”机构形成，可能的机理如下：H_2O → (OH)_(ad) + H~+ + e~- (OH)_(ad) + H_2O → (O)_(ad) + H_3O~+ + e~- 2(O)_(ad) → O_2 [Cr(H_2O)_6]~(3+) + (O)_(ad) → (CrO_3~-)_(ad) + 4H~+ + 4H_2O (CrO_3~-)_(ad) + H_2O → HCrO_4~- + H~+ +e~- 2HCrO_4~- <-> Cr_2O_7~(2-) + H_2O.

二氧化硫的阳极氧化

本文对贵金属电极和钴卟啉膜电极进行了动力学研究，并根据实验数据提出了SO_2在不同电极材料上的反应机理。一、各种电极材料催化性能的比较用循环伏安法得出SO_2在Pt、Au、Pd电极上的氧化在不同电位区有不同的机理。一种是在低电位区SO_2分子的直接放电，另一种是在高电位区表面吸附氧的化学氧化。在贵金属Ir、Ru、Rh电极上，So_2的氧化只有一种途径，即只能通过电极表面吸附氧的化学氧的化学氧化。根据SO_2在贵金属电极上氧化的峰电位得出SO_2在这些电极上的活性次序如下：Au > Ir > Pd，Ru > Pt > Rh SO_2在钴卟啉膜电极上氧化也只有一种途径，但和Ir、Ru、Rh电极不同，不是通过吸附氧的化学氧化，而是SO_2分子的直接放电。二、氧化机理1、SO_2在Au电极上电化学氧化机理 对SO_2在Au电极上电化学过程进行研究，用稳态极化曲线法测得在较负电位区Tafel线性区的斜率为0.049, 可说明SO_2分子第二个电子的脱除为控制步骤；用计时－电流法、计时－电位法求得SO_2在Au电极上电化学氧化的动力学参数R_6~o为10~(-11)数量级。测量不同电位下交流阻抗的频谱，得到两个明显的半园，表明电极过程包括了中间吸附物的形成。根据实验数据推导SO_2在Au电极上可能的电化学氧化机理如下：H_2SO_3 → H_2SO_3ads H_2(SO_3)_(ads) - e → H_2SO_3~+ H_2SO_3~+ +H_2O → HSO_3+H_3O~+ HSO_3 - e → HSO_3~+ HSO_3~+ + 2H_2O → H_2SO_4+H_3O~+ 2. So_2在Ru电极上的氧化机理在贵金属Ru电极上，SO_2通过吸附氧氧化，提出如下如能的反应机理：Ru + H_2O → Ru(OH)_(ads) + H~+ + e Ru(OH)_(ads) + H_2O → RuO_(ads) + H_3O~+ + e RuO_(ads) + SO_2 + H_2O → Ru + H_2SO_4用稳态法测得Tafel斜率为0.055，限制步骤可能为吸附氧氧化SO_2分子。3、SO_2 在Copp/Pt上的氧化机理在非金属钴卟啉膜电极上，SO_2氧化为SO_2分子的直接放电。稳态测量法得到极化曲线的Tafel斜率为0.107 (α=0.54)。表明SO_2分子的第一电子的脱除为控制步骤；计时－电位法表明SO_2在钴卟啉电极上氧化有个前置的化学反应p在不同电位下测量交流阻抗频谱也为两个半园，表明有中间吸附物形成。根据实验数据推导SO_2氧化的可能机理如下：H_2SO_3 → H_2SO_(3_(ads)) H_2SO_(3_(ads)) <-> H~+ + HSO_(3_(ads))~- HSO_(3_(ads))~- - e → HSO_3 HSO_3 - e → HSO_3~+ HSO_3~+ 2H_2O → H_2SO_4 + H_3O~+

EFFECTS OF DEEP CENTERS AT PT SI INTERFACES AND HYDROGENATION

Electrical measurements were combined with surface techniques to study the Pt/Si interfaces at various silicide formation temperatures. Effects of deep centers on the Schottky barrier heights were studied. Hydrogen plasma treatment was used to passivate the impurity/defect centers at the interfaces, and the effects of hydrogenation on the Schottky barrier heights were also examined. Combining our previous study on the Pt/Si interfacial reaction, factors influencing the PtSi/Si Schottky barrier diode are discussed.

Pt/n-GaN肖特基接触的退火行为

在金属有机物气相外延（MOVPE）方法生长的非故意掺杂的n-GaN上用Pt制成了肖特基接触，并在250-650℃范围内对该接触进行退火。通过实验发现，Pt与非故意掺杂n-GaN外延薄膜可以形成较好的肖特基接触，而适当的退火温度可以有效地改善Pt/n-GaN肖特基接触的性质。在该实验条件下，400℃温度下退火后的Pt/n-GaN肖特基接触，势垒高度最大，理想因子最小。在600℃以上温度退火后，该接触特性受到破坏，SEM显示在该温度下，Pt已经在GaN表面凝聚成球，表面形成孔洞。