Features of magnetic resonance in nanoporous carbon with pores filled with transition-metal atoms

Properties of nanoporous carbon (NPC) free of metal atoms and NPC containing atoms of Ni, Co and Pd in their pores are studied by electron spin resonance (ESR). The asymmetrical ESR line with the so-called Dyson line shape points out that charge carriers are responsible for the resonance spectrum in metal-free NPC. Although the amount of Ni, Co, and Pd introduced into nanopores is small, the NPC properties change significantly. A bulk ferromagnetism is observed in the case of NPC with Co and Pd, but not in NPC:Ni. Co atoms in pores of NPC cause the formation of a new material, namely, a disordered ferromagnetic medium with some features in the Co atom distribution. Magnetic properties are strongly temperature-dependent. The temperature dependences of the conductivity and ESR integral intensity in NPC:Ni reveal an exponential growth with the same activation energy. The magnetic resonance spectrum of NPC:Pd consists of four signals for NPC which is produced from SiC. There are one ferromagnetic and three paramagnetic signals belonging to the carbon dangle sp³- and sp²-bonds kind and to the paramagnetic clusters of Pd atoms.     

Palladium clusters in nanoporous carbon samples: Magnetic properties

The magnetic properties of nanoporous carbon samples were studied. The samples were prepared from silicon and boron carbides and contained palladium clusters incorporated into pores. Electron spin resonance (ESR) studies over a wide temperature range (3–100 K) showed that the palladium clusters have a significant effect on the electrical and magnetic characteristics of composite samples. In particular, the ESR spectrum of samples with palladium has a narrow line whose intensity varies with temperature following the Curie law, which indicates the formation of an ensemble of localized spins. Therefore, the hole conductivity of the carbon skeleton is partially compensated. The ESR spectra of C(SiC): Pd have a ferromagnetic resonance line, which suggests that some Pd clusters in pores are magnetically ordered. This line is absent in C(B₄C)B: Pd samples. However, the small variation of the resonance frequencies with temperature indicates that the C(B₄C): Pd samples also contain small magnetic inclusions. Original Russian Text ? B.D. Shanina, A.M. Danishevskiĭ, A.I. Veynger, D.A. Kurdyukov, S.K. Gordeev, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 3, pp. 596–603.     

The catalytic activity of chemisorbed oxygen and carbon monoxide species on palladium and related metals investigated using SCF-Xα-SW calculations

Theoretical calculations of the electronic structure of small clusters modelling the adsorption of carbon monoxide on nickel and palladium and the adsorption of oxygen on palladium and platinum have been carried out using the SCF-Xα-SW method. Our results for the carbon monoxide-nickel clusters are in good agreement with earlier work. Comparing the carbon monoxide-nickel with the carbon monoxide-palladium results, suggests that the ordering of CO-derived cluster orbitals is the same in both cases but that the relative shifts are much different. In addition, the oxygen atom participates more significantly in the cluster containing palladium. An analysis of optical transition energies for Ni₅CO and Pd₅CO clusters is given and discussed in terms of experimental data regarding photodesorption of carbon monoxide. In the case of oxygen atoms in platinum and palldium clusters, we have used two different M₅O geometries: one in which the five metal atoms are in a single plane and oxygen is directly over a single metal atom (Type A) and a second in which the oxygen atom is coordinated to four sur face metal atoms and is directly over a metal atom in the second layer (Type B). The levels calculated for the Pd50 type B cluster are in good agreement with available UPS data. Significant differences in type A and type B clusters are noted for palladium. The results can be correlated with experimental Auger spectroscopic and kinetic data. In particular, the type A oxygen cor relates well with an experimentally observed very reactive species, while type B oxygen cor relates well with a quite unreactive species. These two types also correlate with two species observed by Auger spectroscopy.     

Structure and stability of a silicon cluster on sequential doping with carbon atoms

SiC is a highly stable material in bulk. On the other hand, alloys of silicon and carbon at nanoscale length are interesting from both technological as well fundamental view point and are being currently synthesized by various experimental groups (Truong et. al., 2015 [26]). In the present work, we identify a well-known silicon cluster viz., Si₁₀ and dope it sequentially with carbon atoms. The evolution of electronic structure (spin state and the structural properties) on doping, the charge redistribution and structural properties are analyzed. It is interesting to note that the ground state SiC clusters prefer to be in the lowest spin state. Further, it is seen that carbon atoms are the electron rich centres while silicon atoms are electron deficient in every SiC alloy cluster. The carbon–carbon bond lengths in alloy clusters are equivalent to those seen in fullerene molecules. Interestingly, the carbon atoms tend to aggregate together with silicon atoms surrounding them by donating the charge. As a consequence, very few Si–Si bonds are noted with increasing concentrations of C atoms in a SiC alloy. Physical and chemical stability of doped clusters is studied by carrying out finite temperature behaviour and adsorbing O₂ molecule on Si₉C and Si₈C₂ clusters, respectively.     

Density-functional study of Si<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>C<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 10–15) clusters

Density-functional theory with generalized gradient approximation for the exchange-correlation potential has been used to calculate the structural and electronic properties of Si _n C _n (n = 10–15) clusters. We find that the Si _n C _n clusters prefer cagelike structures. An extensive isomer search shows that the lowest-energy arrangements are those in which the silicon atoms and the carbon atoms form two distinct subunits. It is found that the carbon atoms favor to form fullerene-like structure due to the sp ²-like bond. The silicon atoms are trying to cope with an unfavorable sp ² environment, but distorted tetrahedra still show up somewhere of the cagelike structures. On the basis of the lowest-energy geometries obtained, the binding energy, HOMO–LUMO gap, Mulliken charge, ionization potential and electron affinity of the clusters have been computed and analyzed. An electronic charge transfer from the Si-populated to the C-populated regions is observed.     

Electron spin resonance of nanoporous carbon with embedded cobalt clusters

Cobalt clusters were embedded into a nanoporous carbon powder (with pores about 2 nm in size) prepared from a B₄C carbide powder. Electron spin resonance (ESR) measurements were carried out within a broad temperature range. At all temperatures, the ESR spectrum consisted of two overlapping resonance Lorentzian lines. The temperature dependences of the integrated intensities and linewidths and of the resonance fields were determined. A theoretical analysis of these dependences shows that they can be described in terms of the theory of a disordered magnetic medium with two spin systems with different properties.     

Palladium clusters in nanoporous carbon samples: Structural properties

The structure of nanoporous carbon samples (prepared from silicon and boron carbides) with incorporated palladium clusters was studied. X-ray and electron diffraction studies show that most Pd clusters have an fcc lattice. Using small-angle X-ray scattering measurements, the metal cluster sizes are determined under certain assumptions. The sizes are not very close to the cluster sizes (4–14 nm) that are found from photomicrographs obtained using a transmission electron microscope (TEM). The difference is likely due to the local character of the TEM measurements. Apart from relatively coarse clusters of the sizes indicated above, the samples contain very small clusters, which are smaller in size than a micropore. Such small clusters are particularly large in number in C(SiC)B: Pd, where they are 0.5–0.7 nm in size. The content of small clusters in C(B₄C)B: Pd is substantially lower, and they are somewhat larger in size (1.2–1.6 nm). The possible reasons of the ferromagnetism observed in these samples are discussed. It is assumed that the magnetism may be due to the small clusters, which do not have cubic symmetry. Original Russian Text ? A.M. Danishevskiĭ, R.N. Kyutt, A.A. Sitnikova, B.D. Shanina, D.A. Kurdyukov, S.K. Gordeev, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 3, pp. 604–608.     

Structure of endohedral complexes of carbon nanotubes encapsulated with lithium and sodium

The article provides the results of ab initio calculations employing density functional theory of carbon nanotubes that contain clusters of lithium and sodium atoms. Stable positions of interstitial atoms, the electron density distribution in the system and the density of electronic states are determined. It is shown that the amount of charge transferred from the interstitial atoms in a cluster significantly differs from the corresponding value for a single atom. It is established that the density of electronic states of the system at low concentrations of atoms of the introduced element is determined by the electronic structure of a hollow nanotube, and as the concentration of interstitial atoms increases, this quantity becomes virtually independent on the type of alkali metal (lithium or sodium) and the initial type of the nanotube conductivity.     

Electron delocalization in magnesium clusters grown in supercold helium droplets

The formation of bare clusters from highly reactive metals can be achieved very effectively by the pickup of atoms into superfluid helium droplets. We report on the experimental observation of electronic shells in small magnesium clusters produced by this method. Mass spectra taken under various ionization conditions show steps and outstanding peaks, as well as pronounced minima. The abundance distribution suggests a transition to full electron delocalization which is complete at about 20 atoms. A so-far-not-reported electron reorganization is observed, leading to a novel shell structure.     

AunY(n＝1—9)掺杂团簇的结构和电子性质研究

采用相对论有效原子实势(RECP)近似和密度泛函(B3LYP)方法，选择LANL2DZ基组，优化得到了Au_nY(n＝1—9)二元掺杂团簇稳定的基态结构和电子性质.研究结果表明，掺杂Y原子的Au_nY(n＝1—9)团簇随n的变化，其电离势、电子亲合能和费米能级与Au_n(n＝2—9)一样具有“奇-偶”振荡效应；团簇离子的稳定性具有“幻数”现象，Au₂Y⁺和Au₆Y⁺比其他团簇离子更稳定，与质谱实验结果一致；同一团簇中，团簇最稳定的异构体(基态)是趋于Y原子有最大的邻近的Au原子数. 关键词： Au-Y团簇 密度泛函 平衡几何结构 电子性质     

Ni掺杂对Cu团簇结构稳定性的影响

采用密度泛函理论对Cu_n和Cu_n-1Ni(n=3-14)团簇的结构及稳定性进行研究.结果证明Cu_n(n=3-14)团簇的基态不是密实结构而是类似双平面的构型;计算表明:Ni掺杂增加了铜团簇的稳定性,Cu_nNi(n=2-13)团簇的最稳态结构与单质铜团簇不同而是以形成二十面体为基础的密实结构,Ni原子趋于和尽量多的Cu原子成键而最终陷入笼状团簇的中心;偶数个粒子的团簇具有相对高的稳定性,尤其Cu₃Ni,Cu₇Ni和Cu₉Ni;陷入笼状团簇内部的Ni原子带正电,使得位于表面的Cu原子带负电,从而增加了由这种团簇构成的材料的化学稳定性,如耐腐蚀性等.     

基于密度泛函紧束缚方法的Ge_(10)团簇Mülliken交叠电子布居分析与解离行为研究

随着微电子工业和纳米技术的不断发展,对低维锗材料物理和化学性质的研究正成为研发新型微纳电子器件的基础.采用遗传算法和密度泛函紧束缚方法相结合计算得到Ge_(10)团簇最低能量构型.通过对该团簇内局域原子堆积结构和基于Mülliken电子布居的电子性质分析,发现团簇内两个原子间成键的强弱受原子间距和这两个原子各自近邻原子的状况影响.团簇内部原子上的电子会向团簇外部原子转移.团簇的解离会以分成两个团簇和单个原子的方式进行.当以团簇方式解离时,出现两个Ge_5团簇或一个Ge_3和一个Ge_7团簇.位于团簇小表面上方的原子会首先从团簇解离出来,随后八面体顶点上的原子发生解离.     

First-principles study of palladium atom adsorption on the boron- or nitrogen-doped carbon nanotubes

We have performed first-principles calculation to investigate the adsorption of a single palladium atom on the surface of the pristine and boron- or nitrogen-doped carbon nanotubes (CNTs). The results show that for the adsorption of a single palladium atom on the pristine CNT surface, the most stable site is Bridge1 site above the axial carbon–carbon bond. Either boron- or nitrogen-doped CNTs can assist palladium surface adsorption, but the detailed mechanisms are different. The enhanced palladium adsorption on boron-doped CNT is attributed to the palladium d orbital strongly hybridized with both boron p orbital and carbon p orbital. The enhancement in palladium adsorption on nitrogen-doped CNT results from activating the nitrogen-neighboring carbon atoms due to the large electron affinity of nitrogen. Furthermore, the axial bond is preferred over the zigzag bond for a palladium atom adsorbed on the surface of all three types of CNTs. The most energetically favorable site for a palladium atom adsorbed on three types of CNTs is above the axial boron–carbon bond in boron-doped CNT. The enhancement in palladium adsorption is more significant for the boron-doped CNT than it is for nitrogen-doped CNT with a similar configuration. So we conclude that accordingly, the preferred adsorption site is determined by the competition between the electron affinity of doped and adsorbed atoms and preferred degree of the axial bond over the zigzag bond.     

大尺寸硫醇配体保护金团簇的激发态动力学研究

纳米金团簇作为桥梁连接了金纳米粒子和单个金原子,对于揭示表面等离子激元共振和金属键的来源具有重要意义。有机配体保护的纳米金团簇为理解金纳米粒子从金属性质到非金属性质的转变提供了理想的研究对象,而处于转变区域的金团簇的激发态动力学还尚待研究。在本文中,我们总结了表现出分子性质,并且尺寸较大（大于100 个金原子）的纳米金团簇的激发态动力学,同时将其与表现出金属性质的金纳米粒子的激发态动力学进行比较。本文通过对处于转变区域的金团簇的电子和振动动力学的描述,进一步讨论了其电子结构。对大尺寸纳米金团簇激发态弛豫机理的深入理解,将有助于理解金属纳米团簇和纳米粒子的光学性质,从而进一步推动这一类功能材料的设计和应用。     

Electrical and galvanomagnetic properties of nanoporous carbon samples impregnated with bromine

Nanoporous carbon samples with a large specific surface area can be filled with heavier elements or their compounds, which makes it possible to investigate the interaction of their electronic subsystems with carbon. One of the elements convenient for filling pores of carbon materials is bromine. Impregnation of nanoporous carbon samples with bromine causes the occurrence of the processes of micropore filling, monolayer adsorption, and intercalation. It has been found that samples impregnated with bromine substantially change their electrical and galvanomagnetic properties, and these changes depend on the structure of the samples. It has been shown that, if in the skeleton of a porous carbon sample there is a fraction of graphite clusters, the impregnation of the sample with bromine increases the concentration of charged carriers (holes). But when the sample has a quasi-amorphous structure, the injection of bromine into the sample leads to the appearance of a certain concentration of electrons in addition to charged mobile holes of the initial sample; i.e., the electrical conductivity becomes bipolar. In the former case, bromine molecules intercalate graphite clusters and, since bromine is an acceptor during intercalation of graphite, the hole concentration in the carbon skeleton network increases. In the latter case, bromine molecules can only be adsorbed on pore walls. As a result, the adsorption interaction between the electron shells of bromine molecules and the carbon surface leads to the formation of a donor layer near the surface and to the generation of electrons in the carbon skeleton network.     

BN管状团簇到单壁纳米管的几何结构和电子性质

利用密度泛函理论(DFT),对氮化硼(BN)管状团簇的几何结构、稳定性和电子性质进行了研究.选取合适的BN结构单元作为结构生长基元,采用逐层生长的方式计算得到有限长度、不同截面尺寸的稳定管状团簇.结构中B-N交替排列,结构组成中的四元环和六元环数目均符合一般表达式.计算结果表明,通过适当组装管状团簇以及碳原子掺杂,可以制备出带隙可调的单壁氮化硼纳米管.     

First-Principles Study of Geometries,Stabilities and Electronic Properties of Ca2Sin （n=1-11） Clusters

The equilibrium geometries, relative stabilities, and electronic properties of Ca2Sin （n = 1-11） clusters have been systematically investigated by using the density function theory at the 6-311G （d） level The optimized geometries indicate that the most stable isomers have three-dimensional structures for n = 3-11. The electronic properties of Ca2 Sin （n = 1-11） dusters axe obtained through the analysis of the natural charge population, natural electron configuration, vertical ionization potential, and vertical electron affinity. The results show that the charges in corresponding Ca2Sin clusters transfer from the Ca atoms to the Sin host. Based on the obtained lowest-energy geometries, the size dependence of cluster properties, such as averaged binding energies, fragmentation energies, second-order energy differences, HOMO- LUMO gaps and chemical hardness, are deeply discussed.     

Characteristics of magnetism in nanoporous carbon with palladium clusters

The magnetic properties of samples of nanoporous carbon with palladium clusters prepared from polycrystalline SiC have been studied over a wide range of temperatures. The specific magnetic moment rapidly decreases with increasing temperature. The obtained experimental magnetization curves have a nonlinear character but do not reach saturation even in a field of 10 kOe. Being constructed as functions of H/T, they lie on a universal curve, which suggests superparamagnetism of the system. The experimental curves have a hysteresis. The coercive force, first, decreases with increasing temperature and, then (in the range of 5–20 K), increases, thus forming a notch singularity. On the contrary, the residual magnetization rapidly decreases with increasing temperature. A conclusion on the prevailing contribution of superparamagnetic clusters to the magnetic subsystem is made.     

高氢容量硼氢化锂/碳气凝胶复合材料的制备与性质北大核心CSCD

采用"纳米装填"技术和"熔化渗透"工艺成功制备了氢容量在硼氢化锂质量分数80%以上的硼氢化锂/碳气凝胶复合材料。并用扫描电镜、透射电镜、傅里叶红外透射光谱等手段表征了复合材料的结构与性能。发现硼氢化锂填充了碳气凝胶骨架孔隙的90%以上,形成均匀的复合材料。研究了复合材料的形成机制,发现硼氢化锂先进入碳气凝胶骨架的小孔,再逐渐填充大孔。这有利于材料晶粒的细化,提高吸放氢性能,减少结构缺陷。经放氢动力学测试表明,LiBH4/CRF复合材料的放氢速率是文献中LiBH4与活性炭的球磨样品的5倍。     

无序碳单层的电子结构及氢原子吸附的第一性原理

本研究运用第一性原理计算方法,系统地研究了无序碳单层材料不同位点的电子结构及其析氢性能.计算结果显示无序结构中的C-C键相比于石墨烯中的C-C键在26.7%的范围内有不同程度的拉伸或压缩,使得C原子电荷在-0.17～+0.16个电子范围内变化,导致部分C原子电子局域化.电子的局域化增强了C原子的化学活性,从而表现出了较强的吸附性能.我们发现H原子与C原子的键合及析氢性能与C原子间的键角相关.对于三配位的碳原子,其中三个价电子通过sp~2杂化轨道与最邻近的碳原子结合形成较强的共价键,而余下的一个p_z轨道电子可以与H原子在垂直于原子层的方向形成较弱的化学键.无序结构可以打破三个sp~2杂化轨道的对称性,进而影响p_z轨道与氢的成键.本研究在一定程度上揭示了单层无序碳材料结构-性能的构效关系,为实验上设计特定性能的无序碳功能材料提供理论指导.