STM/STS investigation of carbon nanotubes deposited on Bi2Te3 surface

This paper reports our scanning tunneling microscopy and spectroscopy (STM/STS) study of double-walled and multi-walled carbon nanotubes (CNTs) of different diameter deposited on Bi₂Te₃ (narrow gap semiconductor). The approximate diameter of the studied double-walled and multi-walled CNTs was 2 nm and 8 nm, respectively. Crystalline Bi2Te3 was used as a substrate to enhance the contrast between the CNTs and the substrate in the STS measurements performed to examine peculiarities of CNT morphology, such as junctions, ends or structural defects, in terms of their electronic structure.      

STM/STS investigation of edge structure in epitaxial graphene

In this paper, we have used low temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) to study zigzag or armchair edges of epitaxial graphene on 6H-SiC (0001). The monolayer carbon structures exhibit occasionally one-dimensional ridge (1D) in close vicinity to step edge. This ridge exhibits different edges orientations in armchair–zigzag transition which give rise to different local density of states (LDOS) along this 1D structure. This ridge formation is likely explained by residual compressive in-plane stresses.     

Magnetoconductance of carbon nanotube p-n junctions

The magnetoconductance of p-n junctions formed in clean single wall carbon nanotubes is studied in the geometry where a magnetic field is along the tube axis. For long junctions the low temperature magnetoconductance is anomalously large; the relative change in the conductance becomes of order unity even when the flux through the tube is much smaller than the flux quantum. The magnetoconductance is negative for metallic tubes. For semiconducting and small gap tubes the magnetoconductance is nonmonotonic: positive at small and negative at large fields. The identified magnetoconductance mechanism is relevant to magnetotransport in undoped metallic and small gap tubes in the presence of a long range disorder potential.     

Low-temperature STM/STS investigation of nanostructures created by an STM tip on Au(111) surfaces

The structural stability of rapidly solidified (about 10⁴ K/s) Sn–3.7Ag–0.9Zn eutectic solder was explored by high-temperature annealing. For the as-cast solders, the applied fast cooling rate had a significant influence on the microstructure of the solders. The faster the applied cooling rates, the smaller the β-Sn dendrites. After annealing at 473 K for 20 and 50 h, β-Sn dendrites congregated together into bulk ones for minimizing the interfacial energy, and Ag₃Sn intermetallic compounds (IMCs) as well as ternary Ag–Zn–Sn IMCs segregated on the grain boundary of the β-Sn dendrites. It seems that the coarsening of the β-Sn dendrites in the rapidly solidified specimen brought a significant softening during annealing of the explored Sn–Ag–Zn alloy. Finally, the β-Sn dendrites vanished gradually with increase of the annealing period, which leads to a kind of softening.     

Band-structure modulation in carbon nanotube T junctions

We show that the band structure of metallic carbon nanotubes can be dramatically altered by the local electrostatic field. This is realized by coupling chemically functionalized nanotubes to form T junctions. The bar of the T is the conducting channel and the leg of the T is used for local gating. Transport measurements reveal that an energy gap develops upon application of a local electric field in both devices with or without linker molecules at the junction. We propose that the mechanism of the band gap modulation in the T junctions without linker molecules is the field effect, with the linker molecules introducing additional electromechanical and chemical effects.     

Size effect of spin-polarized transport in FM/Single-walled carbon nanotube/FM junctions

We investigated theoretically in detail the size effect of spin-polarized transport in FM/Single-walled carbon nanotube/FM junctions (FM/SWCNT/FM) consisting of the achiral types of tubes: armchair tubes and zigzag tubes. The results show that the spin-polarized transport has different oscillation behaviors with the junction?s size in these two junctions. And the effect of tunnel magnetoresistance (TMR) in zigzag tube is stronger than that in armchair tubes. For all zigzag tubes when the size exceeds a limit the size effect on TMR disappears and TMR value reaches one maximum 20%. Furthermore, for each family of zigzag tubes, this limit of size increases with increasing tube?s radius. For short zigzag tubes the TMR can be negative at some special sizes. And the negative TMR value can reach −12%$-12\text{\%}$ at the angle π. Finally, except for the zero angel, the obtained results show that for all zigzag tubes the extremum of TMR is at the angle π  . For all armchair tubes, the TMR value has one oscillation of small amplitude with the increase of angle and it has two extrema: the maximum at π/2$\pi /2$ and the minimum at π, respectively.     

Gate-tuned high frequency response of carbon nanotube Josephson junctions

Carbon nanotube Josephson junctions in the open quantum dot limit are fabricated using Pd/Al bilayer electrodes, and exhibit gate-controlled superconducting switching currents. Shapiro voltage steps can be observed under radio frequency current excitations, with a damping of the phase dynamics that strongly depends on the gate voltage. These measurements are described by a standard resistively and capacitively shunted junction model showing that the switching currents from the superconducting to the normal state are close to the critical current of the junction. The effective dynamical capacitance of the nanotube junction is found to be strongly gate dependent, suggesting a diffusive contact of the nanotube.     

Structural deformation and intertube conductance of crossed carbon nanotube junctions

We present a first-principles study of the structure and quantum electronic conductance of junctions consisting of two crossed (5,5) single-walled carbon nanotubes. The structures are determined by constrained minimization of total energy at a given force between the two tubes, simulating the effects of substrate-tube attraction or an applied force. We find that the intertube contact distance is very sensitive to the applied force in the range of 0--10 nN. The intertube conductance is sizable for realistic deformation expected from substrate interaction. The results explain the recent transport data on crossed nanotubes and show that these systems may be potentially useful as electromechanical devices.     

碳纳米管Y形分子结的热导率与热整流现象

基于碳纳米管Y形分子结的结构重构, 通过非平衡分子动力学方法和量子修正, 模拟分析了Y形分子结的热导率和热整流现象. 研究表明: 相对单根完整碳管, Y形分子结在不同温度下导致热导率大约12%–85%的下降; Y结主干向分支方向的导热能力强于分支向主干方向的导热能力; Y结降低热导率的作用随着温度的升高逐渐减小; Y结的热整流效果随着温度的上升先减弱后增强. 关键词： 碳纳米管 热导率 热整流     

Metallic glass electronic structure peculiarities revealed by UHV STM/STS

We present the results of ultrahigh vacuum scanning tunneling microscopy/spectroscopy investigation of metallic glass surface. The topography and electronic structure of Ni_63.5Nb_36.5 have been studied. A great number of clusters with size about 5–10 nm have been found on constant current scanning tunneling microscopy images. The tunneling spectra of normalized tunneling conductivity revealed the energy pseudogap in the vicinity of Fermi energy. For energy values above 0.1 eV the normalized tunneling conductivity changes linearly with increasing of tunneling bias. The obtained results can be understood within suggested theoretical model based on the interplay of elastic electron scattering on random defects and weak intra-cluster Coulomb interaction. The effects of the finite edges of electron spectrum of each cluster have to be taken into account to explain the experimental data. The tunneling conductivity behavior and peculiarities in current images of individual clusters can also be qualitatively analyzed in the framework of suggested model.     

STM/STS study of electronic states in highly underdoped Bi2212

Local density of states (LDOS) and the lattice structure of highly underdoped Bi₂Sr₂CaCu₂O_8+δ with T_c = 22 K and 30 K were investigated by a low temperature scanning tunneling microscope. The modulation structure of the Bi–O surface was strongly depressed in the highly underdoped samples. The depression was observed only in the samples subject to the strong reduction annealing process, suggesting that the strong reduction in excess oxygen could destroy the modulation structure. At a time, patch-like inhomogeneity in the gap map sometimes disappeared, indicating that the existence of excess oxygen has an important role in the patch formation. Analysis on the LDOS with various doping levels showed that there was no crossover energy, which separates a pseudogap and a superconducting gap and is proportional to T_c.     

Effects of topological defects in the semiconductor carbon nanotube intramolecular junctions

Several intramolecular junctions (IMJs) connecting two semiconductor single-wall carbon nanotubes (SWNTs) have been realized by using the layer-divided technique and introducing the pentagon-heptagon topological defects. The atomic structure of each IMJ is optimized with a combination of density-functional theory (DFT) and the universal force field (UFF) method, based upon which a -orbital tight-binding calculation is performed on its electronic properties. Obtained results indicate that different topological defects and their distributions on the interfaces of the IMJs have decisive effects on the electronic properties of the IMJs. The specific geometrical defects control the localized defect states chiefly, while the diameters of the SWNTs on both sides are also related to them. The influence on the experimental observation brought by the choice of the scanning line is also presented by comparing the scanning results performed on the defect side with those on the defect-free side. A new IMJ structure has been found, and it probably reflects the real atomic structures of the semiconductor-semiconductor (S-S) IMJ [Phys. Rev. Lett. 90, 216107 (2003)].Received: 29 November 2003, Published online: 9 April 2004PACS: 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 73.20.At Surface states, band structure, electron density of states - 73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions     

Wave propagation in carbon nanotube intramolecular junctions: finite element calculations

The dispersion of longitudinal and transverse waves in (n,0)–(2n,0) intramolecular junctions (IMJs) are investigated using an atomistic finite element method (FEM). The transient responses of IMJs with different connection types subjected to harmonic incident wave were modelled using three-dimensional elastic beams of carbon bonds and point masses. The linkage between the force-field constants of molecular mechanics and input parameters of beam and mass elements was established through the molecular structural mechanics approach. The wave dispersion simulated by FEM shows good agreement with that of the non-local elastic model in a wide frequency range up to the terahertz region. It is shown that both the microstructure of conical part (connection part) and the coupling of longitudinal vibration and transverse vibration brought by the conicity play important roles in the dispersion of longitudinal and transverse wave in a single-walled IMJ. The amplitude decay of longitudinal wave depended on the distance propagating; the wavelength and the structure in connection part are examined. The results show that the dispersion of the decay of the wave amplitude in IMJ with less pentagon–heptagon defects has a better agreement with analytical results of macroscopic conical shell.     

Photocurrents in nanotube junctions

Photocurrents in nanotube p-n junctions are calculated using a nonequilibrium Green function quantum transport formalism. The short-circuit photocurrent displays band-to-band transitions and photon-assisted tunneling, and has multiple sharp peaks in the infrared, visible, and ultraviolet. The operation of such devices in the nanoscale regime leads to unusual size effects, where the photocurrent scales linearly and oscillates with device length. The oscillations can be related to the density of states in the valence band, a factor that also determines the relative magnitude of the photoresponse for different bands.     

The passivation of atomic scale defects present on III–V semiconductor laser facets: an STM/STS investigation

The work presented in this paper is based on the use of scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS) to study the passivation of atomic scale defect-induced surface states on cleaved III–V (1 1 0) surfaces. This is based on the use of thin Si layers deposited in situ on to the atomically clean surface. The simultaneous STM and STS measurements allowed direct correlation of the structural and electronic properties at the nanoscopic level. The preferential adsorption of Si clusters onto surface defects was achieved using elevated temperature growth on the GaAs(1 1 0) substrate. The STS results clearly indicated local electronic passivation of both step defects and vacancy clusters when the interface is formed at 280 °C. This observation was also confirmed on a macroscopic level using X-ray photoelectron spectroscopy (XPS) under identical conditions. The results are interpreted in terms of the surface bonding of Si with the defect sites. Furthermore, this STM/STS study has been extended to real laser devices where comparable defect features are observed. The implications of defect passivation in nanotechnology are also discussed.     

Defects of SiC nanowires studied by STM and STS

For the first time the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are employed to investigate the morphology and the surface electronic structure of the defective silicon carbide nanowires (SiCNWs). The SiCNWs produced via combustion synthesis route are studied. The STS measurements are performed in the current imaging tunneling spectroscopy mode (CITS) that allows us to determine the correlation between STM topography and the local density of electronic states (LDOS) around the bend of an isolated SiCNW. The measurements reveal fluctuations of LDOS in the vicinity of the defect. The local graphitisation and the inhomogeneous concentration of doping impurities (e.g. nitrogen, oxygen) are considered to explain these fluctuations of metallic-like LDOS in the vicinity of the SiCNW's deformation.     

STM/STS实验中单电子隧道现象的观测

报道在低温(4.2K)条件下利用STM/STS实验方法对过渡族金属二硫族化合物1T-TaSe₂的观测结果,明确得到了1T-TaSe₂中电荷密度波超晶格以及原子晶格的直观图像,并且给出了有关电荷密度波能隙的隧道谱结果,通过选择高纯度Fc以及不锈钢金属材料作为探针,还得到了单个隧道结构中单电子隧道(SET)效应的实验证据,即“库仑阻塞”(Coulomb blocade)效应,并以此说明STM隧道结中探针表面氧化层对实验结果的影响。     

STM/STS observation of ferrocene derivative adduct to C60 on HOPG

The C₆₀ONCFn cycloadduct (Fn=ferrocene) was prepared in the reaction between C₆₀ and ferrocene oxime, the ferrocene derivative was bound to C₆₀ at the 6–6 bond by a heterocyclic oxygen–nitrogen–carbon ring; the compound was stable in air. The compound dissolved in dichloroethane was deposited on HOPG and observed by UHV STM/STS methods. The molecules of C₆₀ONCFn formed several-microns-long straight chains with clearly visible adducted groups pointing to one side of the chain. The STM/STS observations are discussed within the terms of semiempirical quantum chemical molecular modeling.