Inelastic tunneling spectroscopy of gold-thiol and gold-thiolate interfaces in molecular junctions: The role of hydrogen

It is widely believed that when a molecule with thiol (S-H) end groups bridges a pair of gold electrodes, the S atoms bond to the gold and the thiol H atoms detach from the molecule. However, little is known regarding the details of this process, its time scale, and whether molecules with and without thiol hydrogen atoms can coexist in molecular junctions. Here, we explore theoretically how inelastic tunneling spectroscopy (IETS) can shed light on these issues. We present calculations of the geometries, low bias conductances, and IETS of propanedithiol and propanedithiolate molecular junctions with gold electrodes. We show that IETS can distinguish between junctions with molecules having no, one, or two thiol hydrogen atoms. We find that in most cases, the single-molecule junctions in the IETS experiment of Hihath et al. [Nano Lett. 8, 1673 (2008)] had no thiol H atoms, but that a molecule with a single thiol H atom may have bridged their junction occasionally. We also consider the evolution of the IETS spectrum as a gold STM tip approaches the intact S-H group at the end of a molecule bound at its other end to a second electrode. We predict the frequency of a vibrational mode of the thiol H atom to increase by a factor ～2 as the gap between the tip and molecule narrows. Therefore, IETS should be able to track the approach of the tip towards the thiol group of the molecule and detect the detachment of the thiol H atom from the molecule when it occurs.     

电极距离对4,4’-联苯二硫酚分子器件非弹性电子隧穿谱的影响

基于杂化密度泛函理论和格林函数方法, 计算了4,4’-联苯二硫酚分子器件的非弹性电子隧穿谱, 并研究了电极距离对该非弹性电子隧穿谱的影响. 计算结果表明, 非弹性电子隧穿谱随电极距离的改变呈明显不同的特征, 从而表明了分子的非弹性电子隧穿谱技术能够灵敏地反映出分子器件的微观结构. 研究结果显示, 垂直于电极表面的振动模式对非弹性电子隧穿谱具有较大的贡献.     

Compilation of inelastic electron tunnelling spectra of molecules chemisorbed on metal oxides

Inelastic electron tunnelling spectroscopy (IETS) is now a well established technique for the study of molecular adsorbates on metal oxides. It yields vibrational spectra of the adsorbates with high resolution and sensitivity to submonolayer coverage. This compilation brings together in one volume a wide range of results obtained in IETS laboratories around the world. As well as providing a ready reference, after the fashion of the various infrared spectral atlases, it is intended to show the extent of the systems to which IETS has already been applied and point the way to further fruitful areas of study. Adhesion, catalysis, lubrication and corrosion are specific areas which have benefitted from IETS investigations. Some 179 figures display spectra contributed from over 20 different research groups. At the end there is a bibliography of more than 300 references updated to mid-1984.     

Inelastic insights for molecular tunneling pathways: bypassing the terminal groups

As an example of the use of inelastic transport to deduce structure in molecular transport junctions, we compute the orientation dependence of the Inelastic Electron Tunneling (IET) spectrum of the 1-pentane monothiolate. We find that upon increasing the tilting angle of the molecule with respect to the normal to the electrode the spectrum changes as the intensity of some vibrations is enhanced. These differences occur because for higher tilting angles the tunneling path that bypasses the terminal group grows in importance. IETS can therefore be used to establish the molecular orientation in junctions terminating with alkyl chains and to investigate experimentally the relative importance of the available tunneling paths.     

Functionality in single-molecule devices: model calculations and applications of the inelastic electron tunneling signal in molecular junctions

We analyze how functionality could be obtained within single-molecule devices by using a combination of non-equilibrium Green's functions and ab initio calculations to study the inelastic transport properties of single-molecule junctions. First, we apply a full non-equilibrium Green's function technique to a model system with electron-vibration coupling. We show that the features in the inelastic electron tunneling spectra (IETS) of the molecular junctions are virtually independent of the nature of the molecule-lead contacts. Since the contacts are not easily reproducible from one device to another, this is a very useful property. The IETS signal is much more robust versus modifications at the contacts and hence can be used to build functional nanodevices. Second, we consider a realistic model of a organic conjugated molecule. We use ab initio calculations to study how the vibronic properties of the molecule can be controlled by an external electric field which acts as a gate voltage. The control, through the gate voltage, of the vibron frequencies and (more importantly) of the electron-vibron coupling enables the construction of functionality: nonlinear amplification and/or switching is obtained from the IETS signal within a single-molecule device.     

Inelastic tunneling spectroscopy using scanning tunneling microscopy on trans-2-butene molecule: spectroscopy and mapping of vibrational feature

Inelastic tunneling spectroscopy (IETS) measurement using scanning tunneling microscopy (STM) with a commercially available STM set up is presented. The STM-IETS spectrum measured on an isolated trans-2-butene molecule on the Pd(110) shows a clear vibrational feature in d2I/dV2 at the bias voltage of 360 mV and -363 mV, which corresponds to the nu(C-H) mode (d2I/dV2 approximately 10 nA/V2). In addition, we have obtained an image by mapping the vibrational feature of nu(C-H) in d2I/dV2. The image is obtained by scanning the tip on the surface with the feedback loop activated while the modulation voltage is superimposed on the sample voltage. With the method that is readily performable with conventional software, we have clearly differentiated the molecules of trans-2-butene and butadiene through the mapping of the vibrational feature, demonstrating its capability of chemical identification in atomic scale.     

Propensity rules for inelastic electron tunneling spectroscopy of single-molecule transport junctions

Using a perturbative approach to simple model systems, we derive useful propensity rules for inelastic electron tunneling spectroscopy (IETS) of molecular wire junctions. We examine the circumstances under which this spectroscopy (that has no rigorous selection rules) obeys well defined propensity rules based on the molecular symmetry and on the topology of the molecule in the junction. Focusing on conjugated molecules of C(2h) symmetry, semiquantitative arguments suggest that the IETS is dominated by a(g) vibrations in the high energy region and by out of plane modes (a(u) and b(g)) in the low energy region. Realistic computations verify that the proposed propensity rules are strictly obeyed by medium to large-sized conjugated molecules but are subject to some exceptions when small molecules are considered. The propensity rules facilitate the use of IETS to help characterize the molecular geometry within the junction.     

Understanding the inelastic electron-tunneling spectra of alkanedithiols on gold

We present results for a simulated inelastic electron-tunneling spectra (IETS) from calculations using the "gDFTB" code. The geometric and electronic structure is obtained from calculations using a local-basis density-functional scheme, and a nonequilibrium Green's function formalism is employed to deal with the transport aspects of the problem. The calculated spectrum of octanedithiol on gold(111) shows good agreement with experimental results and suggests further details in the assignment of such spectra. We show that some low-energy peaks, unassigned in the experimental spectrum, occur in a region where a number of molecular modes are predicted to be active, suggesting that these modes are the cause of the peaks rather than a matrix signal, as previously postulated. The simulations also reveal the qualitative nature of the processes dominating IETS. It is highly sensitive only to the vibrational motions that occur in the regions of the molecule where there is electron density in the low-voltage conduction channel. This result is illustrated with an examination of the predicted variation of IETS with binding site and alkane chain length.     

分子结的非弹性隧道谱和电子-振动的耦合

研究了分子结的非弹性隧道谱, 给出了基于微扰理论近似的非平衡格林函数. 深入研究了非弹性隧道谱和电子-振动耦合常数的相互关系. 同时, 还计算了OPV和OPE分子结的IETS, 计算结果与有关的实验结果具有很好的可比性.     

An introduction to Inelastic Electron Tunneling Spectroscopy (IETS) and its chemical applications

Inelastic Electron Tunnelling Spectroscopy (IETS) provides information concerning the vibrational and excitation modes present in molecular species adsorbed onto an electrically insulating substrate. Our objective is not to cover the entire literature to date. Rather we hope to stimulate an interest in IETS among chemists by outlining some of the systems of chemical interest already studied; and to provide sufficient background information to guide the construction and operation of an inelastic electron tunnelling spectrometer and interpretation of the data.     

A theoretical rationalization of a total inelastic electron tunneling spectrum: the comparative cases of formate and benzoate on Cu(111)

Inelastic electron tunneling spectroscopy (IETS) performed with the scanning tunneling microscope (STM) has been deemed as the ultimate tool for identifying chemicals at the atomic scale. However, direct IETS-based chemical analysis remains difficult due to the selection rules that await a definite understanding. We present IETS simulations of single formate and benzoate species adsorbed in the same upright bridge geometry on a (111)-cleaved Cu surface. In agreement with measurements on a related substrate, the simulated IET-spectra of formate/Cu(111) clearly resolve one intense C-H stretching mode whatever the tip position in the vicinity of the molecular fragment. At variance, benzoate/Cu(111) has no detectable IET signal. The dissimilar IETS responses of chemically related molecules--formate and benzoate adsorbates--permit us to unveil another factor that complements the selection rules, namely the degree of the vacuum extension of the tunneling active states perturbed by the vibrations. As a consequence, the lack of a topmost dangling bond orbital is entirely detrimental for STM-based inelastic spectroscopy but not for STM elastic imaging.     

Communication: Identification of the molecule-metal bonding geometries of molecular nanowires

Molecular nanowires in which a single molecule bonds chemically to two metal electrodes and forms a stable electrically conducting bridge between them have been studied intensively for more than a decade. However, the experimental determination of the bonding geometry between the molecule and electrodes has remained elusive. Here we demonstrate by means of ab initio calculations that inelastic tunneling spectroscopy (IETS) can determine these geometries. We identify the bonding geometries at the gold-sulfur interfaces of propanedithiolate molecules bridging gold electrodes that give rise to the specific IETS signatures that were observed in recent experiments.     

Vibrations of a single adsorbed organic molecule: anharmonicity matters!

Vibrational spectroscopy is a powerful tool to identify molecules and to characterise their chemical state. Inelastic electron tunnelling spectroscopy (IETS) combined with scanning tunnelling microscopy (STM) allows the application of vibrational analysis to a single molecule. Up to now, IETS was restricted to small species due to the complexity of vibration spectra for larger molecules. We extend the horizon of IETS for both experiment and theory by measuring the STM-IETS spectra of mercaptopyridine adsorbed on the (111) surface of gold and comparing it to theoretical spectra. Such complex spectra with more than 20 lines can be reliably determined and computed leading to completely new insights. Experimentally, the vibrational spectra exhibit a dependence on the specific adsorption site of the molecules. Theoretically, this dependence is only accessible if anharmonic contributions to the interaction potentials are included. These joint experimental and theoretical advances open new perspectives for structure determination of organic adlayers.     

Inelastic electron tunneling spectroscopy and vibrational coupling

We discuss the relationship between the inelastic electron tunneling spectroscopy (IETS) and vibronic coupling constant within the Green's function formalism at a level of perturbation theory approximation. We also compare our results with experimental measurements. Our results can provide insights into the mechanism of active vibronic modes for IETS.     

Effects of intermolecular interaction on inelastic electron tunneling spectra

We have examined the effects of intermolecular interactions on the inelastic electron tunneling spectroscopy (IETS) of model systems: a pair of benzenethiol or a pair of benzenedithiol sandwiched between gold electrodes. The dependence of the IETS on the mutual position of and distance between the paired molecules has been predicted and discussed in detailed. It is shown that, although in most cases, there are clear spectral fingerprints present which allow identification of the actual structures of the molecules inside the junction. Caution must be exercised since some characteristic lines can disappear at certain symmetries. The importance of theoretical simulation is emphasized.     

Chemical imaging of single 4,7,12,15-tetrakis[2.2]paracyclophane by spatially resolved vibrational spectroscopy

Single 4,7,12,15-tetrakis[2.2]paracyclophane were deposited on NiAl(110) surface at 11 K. Two adsorbed species with large and small conductivities were detected by the scanning tunneling microscope (STM). Their vibrational properties were investigated by inelastic electron tunneling spectroscopy (IETS) with the STM. Five vibrational modes were observed for the species with the larger conductivity. The spatially resolved vibrational images for the modes show striking differences, depending on the coupling of the vibrations localized on different functional groups within the molecule to the electronic states of the molecule. The vibrational modes are assigned on the basis of ab initio calculations. No IETS signal is resolved from the species with the small conductivity.     

A statistical approach to inelastic electron tunneling spectroscopy on fullerene-terminated molecules

We report on the vibrational fingerprint of single C(60) terminated molecules in a mechanically controlled break junction (MCBJ) setup using a novel statistical approach manipulating the junction mechanically to address different molecular configurations and to monitor the corresponding vibrational modes. In the IETS spectra, the vibrations of the anchoring C(60) dominate the spectra; thus information on the unit anchored with C(60) to the electrodes is masked by the modes arising from the anchoring groups. However, we have identified the additional modes from the fluorene backbone optically.     

Characterization of metal oxide surfaces and thin semiconductor films by inelastic electron tunneling spectroscopy.

Inelastic electron tunneling spectroscopy (IETS) is a unique surface and interface analytical technique using electron tunneling through a metal/insulator/metal tunneling junction at cryogenic temperatures. It gives the vibrational spectrum of a very thin (nm) insulator film and the adsorbed species on it. The high sensitivity, good resolution, and wide spectral range inherent in IETS enable us to analyze the surface and interface of the insulator in detail. The tunneling junction is a good model system for oxide catalysts, electronic devises, and solid state sensors. Information about the surfaces of alumina and magnesia, the adsorption states and chemical reactions of adsorbed species occurring on these oxides can be obtained through an analysis of the tunneling spectra. The structures and properties of evaporated thin semiconductor films can also be studied. In this review, the surface characterization of alumina and magnesia, the adsorption and surface reactions of organic acids, esters, amides, and nitryls on these oxides, and the characterization of thin evaporated films of Si, Ge, and the oxides are summarized.     

Inelastic electron tunneling spectroscopy

Inelastic electron tunneling spectroscopy (IETS) is a relatively new form of vibrational spectroscopy which is able to address problems previously unsolved by either IR or Raman. It is particularly useful for surface analysis.     

Inelastic electron tunneling spectroscopy of alkane monolayers with dissimilar attachment chemistry to gold

We compare the low-temperature electron transport properties of alkyl monolayers which utilize different attachment strategies to gold. Inelastic electron tunneling spectroscopy (IETS) and current-voltage analysis were performed on molecular junctions incorporating alkyl-dithiocarbamate and alkanethiolate self-assembled monolayers of similar length. Alkyl-dithiocarbamate monolayers were formed by the condensation of dioctylamine or didecylamine with carbon disulfide in anhydrous ethanol and compared to alkanethiolate SAMs of 1-decanethiol and 1-dodecanethiol, respectively. The electron transport properties of each monolayer were examined using magnetically assembled microsphere junctions under high-vacuum conditions at low temperature. IETS was employed to differentiate the films on the basis of vibrational modes which are characteristic of each method of attachment. We use quantum chemical simulations of model compounds to calculate frequency and intensity of predicted signals arising from molecular vibrations to aid in the accurate assignment of the spectra. A qualitative comparison of our devices also reveals an increase in current density when utilizing dithiocarbamate attachment to gold compared to alkanethiolate molecules of similar length.