Dependence of single-molecule junction conductance on molecular conformation

Since it was first suggested that a single molecule might function as an active electronic component, a number of techniques have been developed to measure the charge transport properties of single molecules. Although scanning tunnelling microscopy observations under high vacuum conditions can allow stable measurements of electron transport, most measurements of a single molecule bonded in a metal-molecule-metal junction exhibit relatively large variations in conductance. As a result, even simple predictions about how molecules behave in such junctions have still not been rigorously tested. For instance, it is well known that the tunnelling current passing through a molecule depends on its conformation; but although some experiments have verified this effect, a comprehensive mapping of how junction conductance changes with molecular conformation is not yet available. In the simple case of a biphenyl--a molecule with two phenyl rings linked by a single C-C bond--conductance is expected to change with the relative twist angle between the two rings, with the planar conformation having the highest conductance. Here we use amine link groups to form single-molecule junctions with more reproducible current-voltage characteristics. This allows us to extract average conductance values from thousands of individual measurements on a series of seven biphenyl molecules with different ring substitutions that alter the twist angle of the molecules. We find that the conductance for the series decreases with increasing twist angle, consistent with a cosine-squared relation predicted for transport through pi-conjugated biphenyl systems.     

DNA sequencing technology based on nanopore sensors by theoretical calculations and simulations

DNA sequencing based on nanopore sensors is a promising tool for third-generation sequencing technol- ogy because of its special properties, such as revolutionized speed and low cost. With about two decades of nanopore technology development, the pioneering work has dem- onstrated the ability of nanopores to perform single-mole- cule detection and DNA sequencing. However, the microscopic mechanisms of DNA transport dynamics through nanopores remain largely unknown. Currently, DNA microscopic transport in a nanopore is difficult to characterize and several unexpected experimental obser- vations are equivocal. This limitation can be resolved using theoretical calculations and simulations. These computa- tional methods can monitor the entire dynamic process that DNA undergoes in solution at a single-atom resolution that can accurately unveil the mystery of DNA transport dynamics and predict certain unexpected phenomena. This paper mainly reports the recent applications of computa- tional and simulation methods applied to the study of DNA transport through both biological and synthetic nanopores. We hope the theoretical calculations and simulations of DNA transport through nanopores can benefit the design of DNA sequencing devices.     

Gold nanorod translocation through a solid-state nanopore

Recently, nanopores have been used in an essential technique for detecting single molecule with high sensitivity. The initial application of nanopores to DNA and RNA sequencing has been expanded to sensing pro- teins and nanoparticles, including Bovine serum albumin, silica nanoparticles, polystyrene beads, and others. In our study, for the first time, a positively charged gold nanorod was investigated using a solid-state nanopore device. Various gold nanorods passed through the nanopore with different current blockages and duration times, providing a measurement of the nanorod diameter, length, and charge. Our findings indicate that nanopore sensing might be a new method for characterizing the size, shape, and charge of nanoparticles.     

Ballistic carbon nanotube field-effect transistors

A common feature of the single-walled carbon-nanotube field-effect transistors fabricated to date has been the presence of a Schottky barrier at the nanotube--metal junctions. These energy barriers severely limit transistor conductance in the 'ON' state, and reduce the current delivery capability--a key determinant of device performance. Here we show that contacting semiconducting single-walled nanotubes by palladium, a noble metal with high work function and good wetting interactions with nanotubes, greatly reduces or eliminates the barriers for transport through the valence band of nanotubes. In situ modification of the electrode work function by hydrogen is carried out to shed light on the nature of the contacts. With Pd contacts, the 'ON' states of semiconducting nanotubes can behave like ohmically contacted ballistic metallic tubes, exhibiting room-temperature conductance near the ballistic transport limit of 4e(2)/h (refs 4-6), high current-carrying capability (approximately 25 micro A per tube), and Fabry-Perot interferences at low temperatures. Under high voltage operation, the current saturation appears to be set by backscattering of the charge carriers by optical phonons. High-performance ballistic nanotube field-effect transistors with zero or slightly negative Schottky barriers are thus realized.     

Detection and analysis of DNA recapture through a solid-state nanopore

Motion control of a single molecule through a solid-state nanopore offers a new perspective on detecting and analyzing single biomolecules. Repeat recapture of a single DNA molecule reveals the dynamics in DNA translocation through a nanopore and may significantly increase the signal-to-noise ratio for DNA base distin- guishing. However, the transient current at the moment of voltage reversal prevents the observation of instantly recaptured molecules and invalidates the continuous DNA ping-pong control. We performed and analyzed the DNA translocation and recapture experiment in a silicon nitride solid-state nanopore. Numerical calculation of molecular motion clearly shows the recapture dynamics with different delay times. The prohibited time when the data acquisition system is saturated by the transient current is derived by equivalent circuit analysis and finite element simulation. The COMSOL simulation reveals that the membrane capacitance plays an important role in determining the electric field distribution during the charging process. As a result of the transient charging process, a non-constant driving force pulls the DNA back to nanopores faster than theoretically predicted. The observed long time constant in the transient current trace is explained by the dielectric absorption of the membrane capacitor.     

Investigation of self-assembled protein dimers through an artificial ion channel for DNA sensing

Artificial nanopores have become promising tools for sensing DNA. Here, we report a new technique for sensing DNA through a conical-shaped nanopore embed- ded in track-etched polyethylene terephthalate （PET） membrane. Two different streptavidin-conjugated mono- valent DNA probes were prepared that can bind to two distinct segments （at either end） of the target DNA. The size of target DNA-linked to the two streptavidin-conju- gated monovalent DNA probes is double that of the indi- vidual probes. By precisely controlling the tip diameter of the conical nanopore embedded in the PET polymer, events due to the translocation of the streptavidin-conjugated monovalent DNA probes through the nanopore can be fil- tered and purposely undetected, whereas the current pulses due to the translocation of the target DNA-induced self- assembled complexes can be detected. The two streptavi- din-conjugated DNA probes cannot be linked by multi- mismatched DNA. Therefore, multi-mismatched （non- specific） DNA will not induce any current pulse signatures. The current pulse signatures for the self-assembled com- plex can be used to confirm the presence of the target DNA. The size-dependent detection of self-assembled complexes on the molecular level shows strong promise for the detection of biomolecules without interference from the probes.     

Single-mode heat conduction by photons

The thermal conductance of a single channel is limited by its unique quantum value G(Q), as was shown theoretically in 1983. This result closely resembles the well-known quantization of electrical conductance in ballistic one-dimensional conductors. Interestingly, all particles-irrespective of whether they are bosons or fermions-have the same quantized thermal conductance when they are confined within dimensions that are small compared to their characteristic wavelength. The single-mode heat conductance is particularly relevant in nanostructures. Quantized heat transport through submicrometre dielectric wires by phonons has been observed, and it has been predicted to influence cooling of electrons in metals at very low temperatures due to electromagnetic radiation. Here we report experimental results showing that at low temperatures heat is transferred by photon radiation, when electron-phonon as well as normal electronic heat conduction is frozen out. We study heat exchange between two small pieces of normal metal, connected to each other only via superconducting leads, which are ideal insulators against conventional thermal conduction. Each superconducting lead is interrupted by a switch of electromagnetic (photon) radiation in the form of a DC-SQUID (a superconducting loop with two Josephson tunnel junctions). We find that the thermal conductance between the two metal islands mediated by photons indeed approaches the expected quantum limit of G(Q) at low temperatures. Our observation has practical implications-for example, for the performance and design of ultra-sensitive bolometers (detectors of far-infrared light) and electronic micro-refrigerators, whose operation is largely dependent on weak thermal coupling between the device and its environment.     

正常金属调制层对超导隧道结微分电导的影响

在考虑绝缘层厚度的基础上,运用Bogoliubov-de Gennes(BdG)方程和Blonder-Tinkham-Klapwijk(BTK)理论模型,计算了含有正常金属调制层的铁磁金属/绝缘层/正常金属/s波超导隧道结(FINS结)中的准粒子输运系数和微分电导。计算结果表明,微分电导随正常金属调制层厚度的变化呈周期性振荡,振荡周期与外加偏压有关;当金属调制层厚度较薄时,邻近效应将导致在金属层中有铁磁性和超导性共存的可能性。     

An atomic force microscopy study of coal nanopore structure

Coal nanopore structure is an important factor in understanding the storage and migration of absorbed gas in coal. A new method for studying coal nanopore structures is proposed. This idea is based on the nano-level resolution of atomic force microscopy, which can be employed to observe the structural features of coal nanopores clearly, conduct quantitative three-dimensional measurements and obtain structural parameters. Analysis results show that coal nanopores are mainly metamorphic pores and intermolecul...     

Molecular control over Au/GaAs diodes

The use of molecules to control electron transport is an interesting possibility, not least because of the anticipated role of molecules in future electronic devices. But physical implementations using discrete molecules are neither conceptually simple nor technically straightforward (difficulties arise in connecting the molecules to the macroscopic environment). But the use of molecules in electronic devices is not limited to single molecules, molecular wires or bulk material. Here we demonstrate that molecules can control the electrical characteristics of conventional metal-semiconductor junctions, apparently without the need for electrons to be transferred onto and through the molecules. We modify diodes by adsorbing small molecules onto single crystals of n-type GaAs semiconductor. Gold contacts were deposited onto the modified surface, using a 'soft' method to avoid damaging the molecules. By using a series of multifunctional molecules whose dipole is varied systematically, we produce diodes with an effective barrier height that is tuned by the molecule's dipole moment. These barrier heights correlate well with the change in work function of the GaAs surface after molecular modification. This behaviour is consistent with that of unmodified metal-semiconductor diodes, in which the barrier height can depend on the metal's work function.     

阳极氧化电压对纳米孔阵列阳极氧化铝膜形貌的影响

阳极氧化铝是将金属铝进行阳极氧化处理后，在其表面上形成的一层氧化膜，可对金属铝起到装饰和保护的作用．20世纪末人们发现，铝在适当的阳极氧化条件下可以制得具有纳米孔阵列的阳极氧化铝膜．随着纳米科学的蓬勃发展，纳米孔阵列阳极氧化铝膜由于其具有大的比表面积、高纵横比、大小均     

TLM钛合金表面二氧化钛三维结构纳米薄膜的可控制备及表征

为了提高血管支架医用钛合金表面的亲水性和抗溶血作用,采用阳极氧化工艺在近β型TLM(Ti-25Nb-3Mo-2Sn-3Zr)医用钛合金表面构建了具有纳米孔/纳米管三维结构二氧化钛(TiO2)纳米薄膜,并对其结构和性能进行表征.研究结果表明:通过控制一次阳极氧化电压和时间可获得具有上层为无序纳米孔、中层为有序纳米管、底层为有序纳米孔的三维结构TiO2纳米薄膜,该薄膜为无定形态;而采用二次阳极氧化工艺获得了纳米孔互贯网络的TiO2纳米薄膜,该薄膜表面更加平整,且表面孔隙率大、亲水性好,同时具有比一次阳极氧化薄膜更好的抗溶血作用,这为其应用于血管支架方面打下基础.     

正常金属/dx2-y2+idxy混合波超导隧道结中的准粒子寿命效应

考虑到准粒子的有限寿命效应,在Blonder-Tinkham-Klapwijk（BTK）理论框架下,通过求解Bogoliubov-de Gennes（BdG）方程,计算正常金属／dx^2-y^2＋idxy混合波超导隧道结中的准粒子输运系数与隧道谱．结果表明：准粒子的有限寿命效应不仅能压低隧道谱中的电导峰．而且可以有效地阻止峰的滑移;随着非弹性散射的增强,还会在零偏压处形成一宽峰．     

页岩储层纳米孔气体传输耦合模型新研究

页岩气在纳米孔隙的传输过程中受多种因素影响,包括孔隙尺寸和压力、孔隙壁面粗糙度、孔隙力学反应、吸附诱导膨胀反应以及权重因子等。因此需要综合考虑以上因素以及吸附气分子在孔隙中所占空间对气体流动影响的条件下,厘清页岩气的不同运移机制（表面扩散、滑脱流、Knudsen扩散和黏性流动）在不同孔隙尺寸和压力下对纳米孔中总气体流量的贡献率。首先,对页岩气的不同运移方式进行了物理描述及数学表征,然后,在考虑孔隙壁面粗糙度、孔隙力学反应、吸附诱导膨胀反应和权重因子等因素的条件下,建立页岩气在储层纳米孔中的气体传输耦合数学模型,模型可靠性通过格子Boltzmann方法计算结果验证。研究结果表明,当孔径小于10 nm时,纳米孔的总流量主要由表面扩散流量组成,孔径越小,表面扩散流量越大;当孔径为40~250 nm和低压条件下,滑脱流和Knudsen扩散对气体传输影响较大;当孔径大于10 μm时,纳米孔的总流量主要为黏性流量。     

电极距离和外部电压对“金／有机分子／金”三明治型隧道结的电子传输特性影响

以“金／1,4-二氰基甲苯分子(C6H4(CN)2)／金”隧道结为研究对象,从第一性原理出发,计算了电极距离和外部电压2个因素对隧道结电子隧穿特性的影响。隧道结的开启电压随电极距离的变化不是单调的,从1.278nm到1.298nm,开启电压减小;从1.298nm到1.398nm,开启电压增大。外部电压导致界面处的电荷积累会阻碍电子的隧穿。理论计算的电流、电导曲线和实验曲线符合较好。     

多巴胺在2-氨基-5-巯基-[1,3,4]三氮唑自组装膜电极上的电化学行为研究

利用新合成的巯基试剂2-氨基-5-巯基-[1，3,4]三氮唑在金电极表面进行了首次自组装，用电化学法和扫描电子显微镜对自组装膜电极进行了表征．研究了多巴胺在该自组装膜电极上的电化学行为，发现该自组装膜能有效促进多巴胺在电极与溶液之间的电子传递，表现为-二电子传递的准可逆行为，电极反应速率常数为0．1049cm／s．该自组装膜电极用于多巴胺注射液含量的测定，结果满意．     

苝酰二亚胺衍生物的电子输运性质的第一性原理研究

利用密度泛函理论与非平衡格林函数的第一性原理方法,从理论上研究苝酰二亚胺衍生物PTCDI-[CH2]n(n=0,2,4,6)分子体系的电子输运性质.结果表明:所有的分子结体系结构中都出现了负微分电阻现象,体系的电子输运性质取决于体系中分子的长度.随着CH2数目的增加,分子的长度相应增加,体系的电导呈指数减少;当n≥2时,分子结呈现整流特性;当n=6时,分子结在±2.1 V时整流比高达72.6;这种整流现象归因于分子的非对称结构.     

Ion-beam sculpting at nanometre length scales.

Manipulating matter at the nanometre scale is important for many electronic, chemical and biological advances, but present solid-state fabrication methods do not reproducibly achieve dimensional control at the nanometre scale. Here we report a means of fashioning matter at these dimensions that uses low-energy ion beams and reveals surprising atomic transport phenomena that occur in a variety of materials and geometries. The method is implemented in a feedback-controlled sputtering system that provides fine control over ion beam exposure and sample temperature. We call the method "ion-beam sculpting", and apply it to the problem of fabricating a molecular-scale hole, or nanopore, in a thin insulating solid-state membrane. Such pores can serve to localize molecular-scale electrical junctions and switches and function as masks to create other small-scale structures. Nanopores also function as membrane channels in all living systems, where they serve as extremely sensitive electro-mechanical devices that regulate electric potential, ionic flow, and molecular transport across cellular membranes. We show that ion-beam sculpting can be used to fashion an analogous solid-state device: a robust electronic detector consisting of a single nanopore in a Si3N4 membrane, capable of registering single DNA molecules in aqueous solution.     

Retarding and manipulating of DNA molecules translocation through nanopores

Nanopores are emerging sensitive sensors that can detect and analyze single charged molecule. Nanopores present a promising approach for sequencing human gen- ome below US$1,000 because of its superior performance, such as high throughput and low cost. However, a dominant bottleneck, that is, the high translocation speed of DNA molecules, has to be overcome. This property decreases accuracy of nanopore sensors to the single-base level. In this review, we mainly introduce the recent research works of retarding and manipulating of DNA motion through nanopores by actively control of three forces, which are the driving force, interaction force between nanopore and molecule, and exterior drag force. Lastly, conclusion and further outlook are presented pore-based DNA sequencing on future directions of nano- technology.     

4-ATP自组膜的重构对辣根过氧化物酶固定化的影响

对氨基苯硫酚自组装单分子膜(4-ATPSAMs)具有高度的各向异性,与烷基硫醇相比分子间具有较强的相互作用,以及由于苯环上的离域电子使其具有良好的导电性,因而4-ATPSAMs修饰贵金属表面已引起人们的广泛关注.但是4-ATPSAMs的电化学重构将影响蛋白质在金电极上的固定量.笔者通过制备4-ATP与4-BDT混合硫醇自组膜(MSAMs)以控制4-ATPSAMs的电化学重构,利用电化学技术及傅立叶变换红外光谱研究了4-ATPSAMs的电化学重构对辣根过氧化物酶在金电极上固定化的影响.实验结果表明,MSAMs可有效控制4-ATPSAMs的电化学重构,可为调控界面的物理性质和化学性质提供一种有效手段,对生物传感器和生物芯片的研究与应用具有重要意义.