Santanu K. Maiti
Quantum transport for different models of isomer molecules attached to two semi-infinite leads is studied on the basis of Green's function technique. Electronic transport properties are significantly affected by (a) the relative position of the atoms in these molecules and (b) the molecule-to-lead coupling strength of these molecular bridge systems.
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In contrast to the static operations of conventional semiconductor devices, the dynamic conformational freedom in molecular devices opens up the possibility of using molecules as new types of devices such as a molecular conformational switch or for molecular data storage. Bistable molecules, with e.g. two stable cis and trans isomeric configurations, could provide, once clamped between two electrodes, a switching phenomenon in the nonequilibrium current response. Here, we model molecular switch junctions formed at silicon contacts and demonstrate the potential of tunable molecular switches in …
We report the formation of self-assembled monolayers of a molecular photoswitch (azobenzene-bithiophene derivative, AzBT) on cobalt via a thiol covalent bond. We study the electrical properties of the molecular junctions formed with the tip of a conductive atomic force microscope under ultra-high vacuum. The statistical analysis of the current-voltage curves shows two distinct states of the molecule conductance, suggesting the coexistence of both the trans and cis azobenzene isomers on the surface. The cis isomer population (trans isomer) increases (decreases) upon UV light irradiation. The si…
By molecular dynamics simulations and free energy calculations based on Monte Carlo method, the detailed balance between Pt cluster isomers was investigated. For clusters of n<=50, stationary equilibrium is achieved in 100 ns in the canonical ensemble, while longer time is needed for n>50. Then, a statistical mechanical model was built to evaluate unimolecular isomerization rate and simplify the prediction of isomer formation probability. This model is simpler than transition state theory and can be easily applied on ab initio calculations to predict the lifetime of nanostructures.
To explain recent experiment showing the role of odd fullerenes in formation of abundant fullerene isomers a reactive molecular dynamics (MD) study has been performed. Three types of bond rearrangement reactions are found by MD simulations at 3000 K in odd fullerenes which contain an extra sp atom among all other sp$^2$ atoms. The first type is sto-chastic sp-defect migration analogous to exchange mechanism of adatom migration on a surface. The second type cor-responds to changes in the ring configuration of the sp$^2$-structure assisted by the sp atom which can lead to annealing of seven-memb…
Structure-function relationships constitute an important tool to investigate the fundamental principles of molecular electronics. Most commonly, this involves identifying a potentially important molecular structural element, followed by designing and synthesizing a set of related organic molecules, and finally interpretation of their experimental and/or computational quantum transport properties in the light of this structural element. Though this has been extremely powerful in many instances, we demonstrate here the common need for more nuanced relationships even for relatively simple structu…
We study numerically the dynamics of acetanilide (ACN) molecules placed on a flat surface of a multilayer hexagonal boron nitride structure. We demonstrate that the ACN molecules, being achiral in three dimensions, become chiral after being placed on the substrate. Homochirality of the ACN molecules leads to stable secondary structures stabilized by hydrogen bonds between peptide groups of the molecules. Numerical simulations of systems of such molecules reveal that the structure of the resulting hydrogen-bond chains depends on the isomeric composition of the molecules. If all molecules are ho…
In hydrogen bonded systems, nuclear quantum effects such as zero-point motion and tunneling can significantly affect their material properties through underlying physical and chemical processes. Presently, direct observation of the influence of nuclear quantum effects on the strength of hydrogen bonds with resulting structural and electronic implications remains elusive, leaving opportunities for deeper understanding to harness their fascinating properties. We studied hydrogen-bonded one-dimensional quinonediimine molecular networks which may adopt two isomeric electronic configurations via pr…
Interference Blockade in the Conductance of Organic Molecules
采用Landauer方法结合密度泛函方法研究了二苯乙烯分子的{ \em cis/trans}异构体与扶手椅型单壁碳纳米管连接的电导.对于一个给定的电极排列，两种异构体之间的传输显着差异被发现。对于一个给定的异构体，电导可以通过改变分子电极的相对取向的数量级变化。这两种效应都可以用一个简单的、物理上透明的干涉规则来解释，这表明在这样的分子系统中，通过不同的开关机制来进行直接的电导控制。
In this work, we have implemented the Fisher-Lee formalism to couple non-equilibrium Green functions with tight-binding Density Functional to tackle large molecular systems. This method is used to determine the decay constant of a set of oligomers based on seven different monomers taken from the literature in the non-resonant tunneling regime. Results show good agreement with experimental measurements. The approach is then applied to explore the conformational pattern effect as well as the asymmetry and the strength of coupling with the electrode of bisthienylbenzene oligomers sandwiched betwe…
The 'open' and 'closed' isomers of the diarylethene molecule that can be converted between each other upon photo-excitation are found to have drastically different current-voltage characteristics when sandwiched between two graphene nanoribbons (GNRs). More importantly, when one GNR is metallic and another one is semiconducting, strong rectification behavior of the 'closed' diarylethene isomer with the rectification ratio >10^3 is observed. The surprisingly high rectification ratio originates from the band gap of GNR and the bias-dependent variation of the lowest unoccupied molecular orbital (…
Formation of a "Cluster Molecule" (C20)2 and its thermal stability
用紧束缚方法研究了两个单碳富勒烯形成簇分子（C20）2的可能性。发现了几种（C20）2异构体，其中C20富勒烯通过强共价力结合并保持其身份;实际上，这些C20富勒烯在“簇分子”中扮演“原子”的角色。所谓的开[2+2]异构体具有最小能量。详细分析了它的形成途径和T = 2000 - 4000 K的热稳定性。这种异构体由于C20富勒烯之一的衰变或两个C20富勒烯聚结成C40簇而失去其分子结构。计算出亚稳态开放[2+2]构型的能垒为U = 2 - 5 eV。
We apply a first-principles computational approach to study a light-sensitive molecular switch. The molecule that comprises the switch can convert between a trans and a cis configuration upon photo-excitation. We find that the conductance of the two isomers vary dramatically, which suggests that this system has potential application as a molecular device. A detailed analysis of the band structure of the metal leads and the local density of states of the system reveals the mechanism of the switch.
Theory of Vibrationally Inelastic Electron Transport through Molecular Bridges
研究了通过分子桥连接到两个引线上的振动非弹性电子输运。这项研究是基于一个通用的振动激发模型的共振传输的电子通过分子结。采用电子-分子散射理论的方法，可以精确地计算出分子桥的透射率。通过结的电流近似使用Landauer型公式获得。考虑到不同的参数制度，其中包括一个分子桥的情况下，是弱耦合到引线，导致窄的共振结构，和相反的情况下，与引线的强相互作用所造成的广泛的共振，我们调查的特性影响的相干和耗散振动运动的电子输运。此外，广泛使用的近似，如宽带近似和弹性输运机制的限制的有效性进行了一些详细的调查。
We investigate the properties of clusters formed from two connected $\mathrm{Ge_{m}}$ cage-like clusters, such as experimentally synthesized $\mathrm{Au_3Ge_{18}^{5-}}$, using first-principles DFT methods. We focus particularly on $\mathrm{Au_nGe_{12}^{q-}}$ formed from $\mathrm{Ge_{6}}$, where $n=0-3$ and $q=0,2$. We examine the geometries, electronic structure, and thermal excitations of these clusters using the SIESTA code, where we test stability using short molecular dynamics simulations. Our examination of intercluster bridges between $\mathrm{Ge_{m}}$, formed of either Ge-Ge or Au-Ge bo…
Electron transport through molecular bridge shows novel quantum features. Propogation of electronic wave function through molecular bridge is completely different than individual atomic bridge employed between two contacts. In case of molecular bridge electronic wave propagators interfere and effect conduction through molecular bonding and anti-bonding states.In the present work i showed through simple calculation that interference of electronic wave propagators cause asymmetric propagation of electronic wave through bonding and anti-bonding state. While for hydrogenic molecule these propagato…
We report a full self-consistent ab initio calculation of the conductance of a diarylethene-based molecular switch with two graphene electrodes. Our result show the contributions of the resonant states of the molecule, of the electrode density of states, and of graphene unique features such as edge states. The conductivities are found to be significantly different for the two photochromic isomers at zero and finite applied bias. Further we point out the possibility of causing the switching by the application of a large potential difference between the two electrodes.
Stabilizing single atom contacts by molecular bridge formation
金-分子-金结可以通过在含有二硫醇化分子的溶液中小心地断开金线来形成。令人惊讶的是，很少有人了解桥形成过程的机械细节，特别是二硫醇分子本身所起的作用。我们提出，烷二硫醇分子已经形成的金电极之间的原子金结被打破之前的桥梁。这导致单原子金结的稳定，如实验所观察到的。我们的数据可以在一个简单的弹簧模型中理解。
When the interaction between a molecular system and confined light modes in an optical or plasmonic cavity is strong enough to overcome the dissipative process, hybrid light-matter states (polaritons) become the fundamental excitations in the system. The mixing between the light and matter characters modifies the photophysical and photochemical properties. Especially, it was reported that these polaritons can be employed to control photochemical reactions, charge and energy transfer, and other processes. In addition, according to recent studies, vibrational strong coupling can be employed to r…
Electronic Transport in Fullerene C20 Bridge Assisted by Molecular Vibrations
利用Keldysh非平衡格林函数技术结合紧束缚分子动力学方法，以最小的富勒烯C20桥为研究对象，研究了分子振动对电子输运的影响.大的不连续的步骤出现在微分电导时，所施加的偏置电压匹配特定的振动能量。台阶的大小随振动模式的不同而有很大的变化，并且除了电子振动耦合的强度外，还取决于局部电子态。在此基础上，提出了一种通过调节栅压来控制分子运动的新方法。
Electron transport characteristics are investigated through some molecular chains attached to two non-superconducting electrodes by the use of Green's function method. Here we do parametric calculations based on the tight-binding formulation to characterize the electron transport through such bridge systems. The transport properties are significantly influenced by (a) the length of the molecular chain and (b) the molecule-to-electrodes coupling strength and here we focus are results in these aspects. In this context we also discuss the steady state current fluctuations, the so-called shot nois…
Nonequilibrium electron charging in carbon-nanotube-based molecular bridges
我们的证据的重要性，电子充电的非平衡条件下的碳纳米管为基础的分子桥，使用自洽的格林函数方法与扩展的休克尔哈密顿量和三维泊松求解器。我们的分析表明，这样的功能是高度依赖于手性的碳纳米管以及接触金属的类型，空调在非广义的方式系统的传导机制。基于其对输运的影响，我们认为，自洽性是必不可少的半导体纳米管的电流-电压计算，而在金属的情况下不太重要。
Electronic transport and vibrational modes in the smallest molecular bridge: H2 in Pt nanocontacts
我们提出了一个最先进的第一原理分析的电子传输在Pt纳米接触在H2的存在下，这是最近报道的Smit等人在自然419，906（2002）。我们的研究结果表明，在最后阶段的Pt纳米接触的破坏两个基本形式的桥梁涉及H可以出现。我们的主张是，与Smit等人。在G约2e^2/h处的主电导直方图峰不是由于分子H2，而是由于H2分子解离的复合物Pt 2 H2。与实验结果相比，第一原理振动分析也支持我们的主张。
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