Artem Mishchenko | The University of Manchester (original) (raw)

Papers by Artem Mishchenko

ACS Nano, 2014

Understanding of the electrochemical properties of graphene, especially the electron transfer kin... more Understanding of the electrochemical properties of graphene, especially the electron transfer kinetics of a redox reaction between the graphene surface and a molecule, in comparison to graphite or other carbon-based materials, is essential for its potential in energy conversion and storage to be realized.

Nature Nanotechnology, 2013

The celebrated electronic properties of graphene 1,2 have opened way for materials just one-atom-... more The celebrated electronic properties of graphene 1,2 have opened way for materials just one-atom-thick 3 to be used in the post-silicon electronic era 4 . An important milestone was the creation of heterostructures based on graphene and other two-dimensional (2D) crystals, which can be assembled in 3D stacks with atomic layer precision 5-7 . These layered structures have already led to a range of fascinating physical phenomena , and also have been used in demonstrating a prototype field effect tunnelling transistor 12 -a candidate for post-CMOS technology. The range of possible materials which could be incorporated into such stacks is very large. Indeed, there are many other materials where layers are linked by weak van der Waals forces, which can be exfoliated and combined together to create novel highly-tailored heterostructures. Here we describe a new generation of field effect vertical tunnelling transistors where 2D tungsten disulphide serves as an atomically thin barrier between two layers of either mechanically exfoliated or CVD-grown graphene. Our devices have unprecedented current modulation exceeding one million at room temperature and can also operate on transparent and flexible substrates.

Topics in Current Chemistry, 2011

Charge transport characteristics in metal-metal nanocontacts and single molecular junctions were ... more Charge transport characteristics in metal-metal nanocontacts and single molecular junctions were studied at electrified solid-liquid interfaces employing a scanning tunneling microscope-based break junction technique, in combination with macroscopic electrochemical methods, in non-conducting solvents and in an electrochemical environment. We aim to demonstrate recent attempts in developing fundamental relationships between molecular structure, charge transport characteristics, and nanoscale electrochemical concepts. After an introduction and brief description of the experimental methodology, a case study on the electrical and mechanical properties of gold atomic contacts in aqueous electrolytes is presented. In experiments with alkanedithiol and a,o-biphenyldithiol molecular junctions the role of sulfur-gold couplings and molecular conformation, such as gauche defects in alkyl chains and the torsion angle between two phenyl rings, are addressed. The combination with quantum chemistry calculations enabled a detailed molecular-level understanding of the electronic structure and transport characteristics of both systems. Employing the concept of "electrolyte gating" to 4,4 0 -bipyridine and redox-active molecules, such as perylene bisimide derivatives, the construction of "active" symmetric and asymmetric molecular junctions with transistor-and diode-like behavior upon polarization in an electrochemical environment will be demonstrated. The latter experimental data could be represented quantitatively by the Kutznetsov/Ulstrup model, assuming a two-step electron transfer with partial vibration relaxation. Finally, we show that (individual) surfaceimmobilized gold clusters within the quantum-confined size range exhibit features of locally addressable multistate electronic switching upon electrolyte gating, which appears to be reminiscent of a sequential charging through several redox states. The examples addressed here demonstrate the uniqueness and capabilities of an electrochemical approach for the fundamental understanding and for potential applications in nano-and molecular electronics.

Journal of Physics-condensed Matter, 2012

We describe a new setup for simultaneous measurements of force and current in conductive nanocont... more We describe a new setup for simultaneous measurements of force and current in conductive nanocontacts in a liquid environment with a high sampling rate and resolution. A lab-built current-to-voltage converter allows measurements of the current over seven orders of magnitude. As examples, we studied conductances and mechanical forces upon formation and breaking of gold atomic contacts and of two molecular junctions containing 1,2-di(4-pyridyl)ethyne (M1) and 1,4-di(4-pyridyl)buta-1,3-diyne (M2). We found that the forces required to deform or break gold atomic contacts depend critically on the surrounding medium. Further, they show non-linear behaviour in dependence of the number N of gold atoms detached. The electromechanical properties of the two types of molecular junctions upon stretching were analysed by correlating breaking forces with simultaneously measured junction conductances. A rather complex behaviour in a wide range of forces was discovered. Comparison of the current-probe atomic force microscopy experiments on the rupture of molecular junctions with STM-based break junction experiments enables the assignment of breaking forces of molecular junctions to the corresponding junction conductances.

Nano Letters, 2010

The conductance of a family of biphenyl-dithiol derivatives with conformationally fixed torsion a... more The conductance of a family of biphenyl-dithiol derivatives with conformationally fixed torsion angle was measured using the scanning tunneling microscopy (STM)-break-junction method. We found that it depends on the torsion angle between two phenyl rings; twisting the biphenyl system from flat ( ) 0°) to perpendicular ( ) 90°) decreased the conductance by a factor of 30. Detailed calculations of transport based on density functional theory and a two level model (TLM) support the experimentally obtained cos 2 correlation between the junction conductance G and the torsion angle . The TLM describes the pair of hybridizing highest occupied molecular orbital (HOMO) states on the phenyl rings and illustrates that the π-π coupling dominates the transport under "off-resonance" conditions where the HOMO levels are well separated from the Femi energy. * Towhomcorrespondenceshouldbeaddressed.

Journal of Physics-condensed Matter, 2008

We report a scanning tunneling microscopy (STM) experiment in an electrochemical environment whic... more We report a scanning tunneling microscopy (STM) experiment in an electrochemical environment which studies a prototype molecular switch. The target molecules were perylene tetracarboxylic acid bisimides modified with pyridine (P-PBI) and methylthiol (T-PBI) linker groups and with bulky tert-butyl-phenoxy substituents in the bay area. At a fixed bias voltage, we can control the transport current through a symmetric molecular wire Au|P-PBI(T-PBI)|Au by variation of the electrochemical 'gate' potential. The current increases by up to two orders of magnitude. The conductances of the P-PBI junctions are typically a factor 3 larger than those of T-PBI. A theoretical analysis explains this effect as a consequence of shifting the lowest unoccupied perylene level (LUMO) in or out of the bias window when tuning the electrochemical gate potential VG. The difference in on/off ratios reflects the variation of hybridization of the LUMO with the electrode states with the anchor groups. I T -E S(T) curves of asymmetric molecular junctions formed between a bare Au STM tip and a T-PBI (P-PBI) modified Au(111) electrode in an aqueous electrolyte exhibit a pronounced maximum in the tunneling current at −0.740, which is close to the formal potential of the surface-confined molecules. The experimental data were explained by a sequential two-step electron transfer process.

Journal of the American Chemical Society, 2012

Employing a scanning tunneling microscopy based beak junction technique and mechanically controll... more Employing a scanning tunneling microscopy based beak junction technique and mechanically controlled break junction experiments, we investigated tolane (diphenylacetylene)-type single molecular junctions having four different anchoring groups (SH, pyridyl (PY), NH 2 , and CN) at a solid/liquid interface. The combination of current−distance and current−voltage measurements and their quantitative statistical analysis revealed the following sequence for junction formation probability and stability: PY > SH > NH 2 > CN. For all single molecular junctions investigated, we observed the evolution through multiple junction configurations, with a particularly well-defined binding geometry for PY. The comparison of density functional theory type model calculations and molecular dynamics simulations with the experimental results revealed structure and mechanistic details of the evolution of the different types of (single) molecular junctions upon stretching quantitatively.

Beilstein Journal of Nanotechnology, 2011

π-Conjugation plays an important role in charge transport through single molecular junctions. We ... more π-Conjugation plays an important role in charge transport through single molecular junctions. We describe in this paper the construction of a mechanically controlled break-junction setup (MCBJ) equipped with a highly sensitive log I-V converter in order to measure ultralow conductances of molecular rods trapped between two gold leads. The current resolution of the setup reaches down to 10 fA. We report single-molecule conductance measurements of an anthracene-based linearly conjugated molecule (AC), of an anthraquinone-based cross-conjugated molecule (AQ), and of a dihydroanthracene-based molecule (AH) with a broken conjugation. The quantitative analysis of complementary current-distance and current-voltage measurements revealed details of the influence of π-conjugation on the single-molecule conductance.

Nano Letters, 2010

The conductance of a family of biphenyl-dithiol derivatives with conformationally fixed torsion a... more The conductance of a family of biphenyl-dithiol derivatives with conformationally fixed torsion angle was measured using the scanning tunneling microscopy (STM)-break-junction method. We found that it depends on the torsion angle between two phenyl rings; twisting the biphenyl system from flat ( ) 0°) to perpendicular ( ) 90°) decreased the conductance by a factor of 30. Detailed calculations of transport based on density functional theory and a two level model (TLM) support the experimentally obtained cos 2 correlation between the junction conductance G and the torsion angle . The TLM describes the pair of hybridizing highest occupied molecular orbital (HOMO) states on the phenyl rings and illustrates that the π-π coupling dominates the transport under "off-resonance" conditions where the HOMO levels are well separated from the Femi energy. * Towhomcorrespondenceshouldbeaddressed.

Nano Letters, 2012

Raman spectroscopy is able to probe disorder in graphene through defect-activated peaks. It is of... more Raman spectroscopy is able to probe disorder in graphene through defect-activated peaks. It is of great interest to link these features to the nature of disorder. Here we present a detailed analysis of the Raman spectra of graphene containing different type of defects. We found that the intensity ratio of the D and D' peak is maximum (∼13) for sp(3)-defects, it decreases for vacancy-like defects (∼7), and it reaches a minimum for boundaries in graphite (∼3.5). This makes Raman Spectroscopy a powerful tool to fully characterize graphene.

Macromolecules, 2012

ABSTRACT The supramolecular assembly of amphiphilic oligopyrenotide building blocks (covalently l... more ABSTRACT The supramolecular assembly of amphiphilic oligopyrenotide building blocks (covalently linked heptapyrene, Py7) is studied by atomic force microscopy (AFM) in combination with optical spectroscopy. The assembly process is triggered in a controlled manner by increasing the ionic strength of the aqueous oligomer solution. Cooperative noncovalent interactions between individual oligomeric units lead to the formation of DNA-like supramolecular polymers. We also show that the terminal attachment of a single cytidine nucleotide to the heptapyrenotide (Py7-C) changes the association process from a cooperative (nucleation–elongation) to a noncooperative (isodesmic) regime, suggesting a structure misfit between the cytidine and the pyrene units. We also demonstrate that AFM enables the identification and characterization of minute concentrations of the supramolecular products, which was not accessible by conventional optical spectroscopy.

Journal of the American Chemical Society, 2011

We present a combined experimental and theoretical study of the electronic transport through sing... more We present a combined experimental and theoretical study of the electronic transport through single-molecule junctions based on nitrile-terminated biphenyl derivatives. Using a scanning tunneling microscope-based break-junction technique, we show that the nitrile-terminated compounds give rise to well-defined peaks in the conductance histograms resulting from the high selectivity of the N-Au binding. Ab initio calculations have revealed that the transport takes place through the tail of the LUMO. Furthermore, we have found both theoretically and experimentally that the conductance of the molecular junctions is roughly proportional to the square of the cosine of the torsion angle between the two benzene rings of the biphenyl core, which demonstrates the robustness of this structure-conductance relationship.

Journal of Physics: Condensed Matter, 2012

We describe a new setup for simultaneous measurements of force and current in conductive nanocont... more We describe a new setup for simultaneous measurements of force and current in conductive nanocontacts in a liquid environment with a high sampling rate and resolution. A lab-built current-to-voltage converter allows measurements of the current over seven orders of magnitude. As examples, we studied conductances and mechanical forces upon formation and breaking of gold atomic contacts and of two molecular junctions containing 1,2-di(4-pyridyl)ethyne (M1) and 1,4-di(4-pyridyl)buta-1,3-diyne (M2). We found that the forces required to deform or break gold atomic contacts depend critically on the surrounding medium. Further, they show non-linear behaviour in dependence of the number N of gold atoms detached. The electromechanical properties of the two types of molecular junctions upon stretching were analysed by correlating breaking forces with simultaneously measured junction conductances. A rather complex behaviour in a wide range of forces was discovered. Comparison of the current-probe atomic force microscopy experiments on the rupture of molecular junctions with STM-based break junction experiments enables the assignment of breaking forces of molecular junctions to the corresponding junction conductances.

European Journal of Organic Chemistry, 2009

The design and synthesis of the macrocyclic turnstile 1 comprising a terminally sulfur-functional... more The design and synthesis of the macrocyclic turnstile 1 comprising a terminally sulfur-functionalized molecular rod and a redox-active catechol subunit is described. The shape-persistent macrocyclic scaffold consists of alternating arylene and ethynylene units. A freely rotating 2,6-diethynyl-catechol subunit is clamped between both terminal arylene subunits as molecular turnstile. While the electrochemical switching between the catechol and the quinone form of this catechol subunit is displayed by cyclic voltammetry, conformational rearrangements by favoring and disfavoring the formation of intramolecular hydrogen bonds are the subject of current investigations. Terminal acetyl-protected sulfur anchor groups enabled the immobilization of the macrocycle between an Au tip and an Au substrate of a STM set-up. Preliminary single-molecule transport investigations of the turnstile 1 display comparable values as for the parent molecular rod. An electrochemically-controlled single-molecule transport experiment to investigate redox-state-dependent transport properties is currently under way.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Dalton Transactions, 2014

Atomically thin layers of materials, which are just a few atoms in thickness, present an attracti... more Atomically thin layers of materials, which are just a few atoms in thickness, present an attractive option for future electronic devices. Herein we characterize, optically and electronically, atomically thin tungsten disulphide (WS 2 ), a layered semiconductor. We provide the distinctive Raman and photoluminescence signatures for single layers, and prepare field-effect transistors where atomically thin WS 2 serves as the conductive channel. The transistors present mobilities μ = 10 cm 2 V −1 s −1 and exhibit ON/OFF ratios exceeding 100 000. Our results show that WS 2 is an attractive option for applications in electronic and optoelectronic devices and pave the way for further studies in this two-dimensional material. Fig. 3 (a) Optical image of a WS 2 transistor. (b) Logarithmic conductance as a function of gate voltage with I on /I off = 100 000 at V b = 0.1 V. Inset: drain current I ds versus drain voltage V ds for various gate voltages.

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2007

Recently, we proposed a new four-parametric MGAB equation (modified Guggenheim-Anderson-De Boer e... more Recently, we proposed a new four-parametric MGAB equation (modified Guggenheim-Anderson-De Boer equation, GAB) for fitting of experimental isotherms of vapor-phase sorption on geosorbents. In the present study, the theoretical meaning of MGAB equation was elucidated. For this purpose, the properties of the equation of multilayer adsorption on a heterogeneous surface with local characteristics described by GAB model and Zeldovitsh-Roginskiy energy distribution function ρ(ε) were derived and the satisfaction of MGAB equation with these properties was shown. MGAB model has two parameters connected with GAB model: v m is the monolayer sorption capacity and k is the constant of the multilayer sorption, as well as two parameters of sorption energy distribution function ρ(ε): α, characterizing heterogeneity of ρ(ε) and C 0 , reflecting energy baseline. The influence of parameters α and C 0 on the shape of both ρ(ε) and model sorption isotherm was established. The applicability of MGAB model for the description of surface heterogeneity of a natural sorbent by example of 13 different sorbate-sorbent systems dried and moistened has been discussed. For these systems equilibrium sorption isotherms were measured, corresponding energy distribution functions ρ(ε) were calculated, and obtained α values were compared. On the whole, the differences in α values responded to the real tendencies of heterogeneity changes in these systems.

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2006

The approximation of isotherms of vapor-phase sorption of organic compounds (OC) is a long-standi... more The approximation of isotherms of vapor-phase sorption of organic compounds (OC) is a long-standing problem. It is especially difficult to solve in case of natural heterogeneous sorbents (soils, sediments, aquifers, etc.) since these sorbents contain sites with different sorption activity. The aim of this work was the statistical evaluation of the adequacy of the approximation of isotherms of vapor-phase sorption

CHIMIA International Journal for Chemistry, 2010

ACS Nano, 2014

Understanding of the electrochemical properties of graphene, especially the electron transfer kin... more Understanding of the electrochemical properties of graphene, especially the electron transfer kinetics of a redox reaction between the graphene surface and a molecule, in comparison to graphite or other carbon-based materials, is essential for its potential in energy conversion and storage to be realized.

Nature Nanotechnology, 2013

The celebrated electronic properties of graphene 1,2 have opened way for materials just one-atom-... more The celebrated electronic properties of graphene 1,2 have opened way for materials just one-atom-thick 3 to be used in the post-silicon electronic era 4 . An important milestone was the creation of heterostructures based on graphene and other two-dimensional (2D) crystals, which can be assembled in 3D stacks with atomic layer precision 5-7 . These layered structures have already led to a range of fascinating physical phenomena , and also have been used in demonstrating a prototype field effect tunnelling transistor 12 -a candidate for post-CMOS technology. The range of possible materials which could be incorporated into such stacks is very large. Indeed, there are many other materials where layers are linked by weak van der Waals forces, which can be exfoliated and combined together to create novel highly-tailored heterostructures. Here we describe a new generation of field effect vertical tunnelling transistors where 2D tungsten disulphide serves as an atomically thin barrier between two layers of either mechanically exfoliated or CVD-grown graphene. Our devices have unprecedented current modulation exceeding one million at room temperature and can also operate on transparent and flexible substrates.

Topics in Current Chemistry, 2011

Charge transport characteristics in metal-metal nanocontacts and single molecular junctions were ... more Charge transport characteristics in metal-metal nanocontacts and single molecular junctions were studied at electrified solid-liquid interfaces employing a scanning tunneling microscope-based break junction technique, in combination with macroscopic electrochemical methods, in non-conducting solvents and in an electrochemical environment. We aim to demonstrate recent attempts in developing fundamental relationships between molecular structure, charge transport characteristics, and nanoscale electrochemical concepts. After an introduction and brief description of the experimental methodology, a case study on the electrical and mechanical properties of gold atomic contacts in aqueous electrolytes is presented. In experiments with alkanedithiol and a,o-biphenyldithiol molecular junctions the role of sulfur-gold couplings and molecular conformation, such as gauche defects in alkyl chains and the torsion angle between two phenyl rings, are addressed. The combination with quantum chemistry calculations enabled a detailed molecular-level understanding of the electronic structure and transport characteristics of both systems. Employing the concept of "electrolyte gating" to 4,4 0 -bipyridine and redox-active molecules, such as perylene bisimide derivatives, the construction of "active" symmetric and asymmetric molecular junctions with transistor-and diode-like behavior upon polarization in an electrochemical environment will be demonstrated. The latter experimental data could be represented quantitatively by the Kutznetsov/Ulstrup model, assuming a two-step electron transfer with partial vibration relaxation. Finally, we show that (individual) surfaceimmobilized gold clusters within the quantum-confined size range exhibit features of locally addressable multistate electronic switching upon electrolyte gating, which appears to be reminiscent of a sequential charging through several redox states. The examples addressed here demonstrate the uniqueness and capabilities of an electrochemical approach for the fundamental understanding and for potential applications in nano-and molecular electronics.

Journal of Physics-condensed Matter, 2012

We describe a new setup for simultaneous measurements of force and current in conductive nanocont... more We describe a new setup for simultaneous measurements of force and current in conductive nanocontacts in a liquid environment with a high sampling rate and resolution. A lab-built current-to-voltage converter allows measurements of the current over seven orders of magnitude. As examples, we studied conductances and mechanical forces upon formation and breaking of gold atomic contacts and of two molecular junctions containing 1,2-di(4-pyridyl)ethyne (M1) and 1,4-di(4-pyridyl)buta-1,3-diyne (M2). We found that the forces required to deform or break gold atomic contacts depend critically on the surrounding medium. Further, they show non-linear behaviour in dependence of the number N of gold atoms detached. The electromechanical properties of the two types of molecular junctions upon stretching were analysed by correlating breaking forces with simultaneously measured junction conductances. A rather complex behaviour in a wide range of forces was discovered. Comparison of the current-probe atomic force microscopy experiments on the rupture of molecular junctions with STM-based break junction experiments enables the assignment of breaking forces of molecular junctions to the corresponding junction conductances.

Nano Letters, 2010

The conductance of a family of biphenyl-dithiol derivatives with conformationally fixed torsion a... more The conductance of a family of biphenyl-dithiol derivatives with conformationally fixed torsion angle was measured using the scanning tunneling microscopy (STM)-break-junction method. We found that it depends on the torsion angle between two phenyl rings; twisting the biphenyl system from flat ( ) 0°) to perpendicular ( ) 90°) decreased the conductance by a factor of 30. Detailed calculations of transport based on density functional theory and a two level model (TLM) support the experimentally obtained cos 2 correlation between the junction conductance G and the torsion angle . The TLM describes the pair of hybridizing highest occupied molecular orbital (HOMO) states on the phenyl rings and illustrates that the π-π coupling dominates the transport under "off-resonance" conditions where the HOMO levels are well separated from the Femi energy. * Towhomcorrespondenceshouldbeaddressed.

Journal of Physics-condensed Matter, 2008

We report a scanning tunneling microscopy (STM) experiment in an electrochemical environment whic... more We report a scanning tunneling microscopy (STM) experiment in an electrochemical environment which studies a prototype molecular switch. The target molecules were perylene tetracarboxylic acid bisimides modified with pyridine (P-PBI) and methylthiol (T-PBI) linker groups and with bulky tert-butyl-phenoxy substituents in the bay area. At a fixed bias voltage, we can control the transport current through a symmetric molecular wire Au|P-PBI(T-PBI)|Au by variation of the electrochemical 'gate' potential. The current increases by up to two orders of magnitude. The conductances of the P-PBI junctions are typically a factor 3 larger than those of T-PBI. A theoretical analysis explains this effect as a consequence of shifting the lowest unoccupied perylene level (LUMO) in or out of the bias window when tuning the electrochemical gate potential VG. The difference in on/off ratios reflects the variation of hybridization of the LUMO with the electrode states with the anchor groups. I T -E S(T) curves of asymmetric molecular junctions formed between a bare Au STM tip and a T-PBI (P-PBI) modified Au(111) electrode in an aqueous electrolyte exhibit a pronounced maximum in the tunneling current at −0.740, which is close to the formal potential of the surface-confined molecules. The experimental data were explained by a sequential two-step electron transfer process.

Journal of the American Chemical Society, 2012

Employing a scanning tunneling microscopy based beak junction technique and mechanically controll... more Employing a scanning tunneling microscopy based beak junction technique and mechanically controlled break junction experiments, we investigated tolane (diphenylacetylene)-type single molecular junctions having four different anchoring groups (SH, pyridyl (PY), NH 2 , and CN) at a solid/liquid interface. The combination of current−distance and current−voltage measurements and their quantitative statistical analysis revealed the following sequence for junction formation probability and stability: PY > SH > NH 2 > CN. For all single molecular junctions investigated, we observed the evolution through multiple junction configurations, with a particularly well-defined binding geometry for PY. The comparison of density functional theory type model calculations and molecular dynamics simulations with the experimental results revealed structure and mechanistic details of the evolution of the different types of (single) molecular junctions upon stretching quantitatively.

Beilstein Journal of Nanotechnology, 2011

π-Conjugation plays an important role in charge transport through single molecular junctions. We ... more π-Conjugation plays an important role in charge transport through single molecular junctions. We describe in this paper the construction of a mechanically controlled break-junction setup (MCBJ) equipped with a highly sensitive log I-V converter in order to measure ultralow conductances of molecular rods trapped between two gold leads. The current resolution of the setup reaches down to 10 fA. We report single-molecule conductance measurements of an anthracene-based linearly conjugated molecule (AC), of an anthraquinone-based cross-conjugated molecule (AQ), and of a dihydroanthracene-based molecule (AH) with a broken conjugation. The quantitative analysis of complementary current-distance and current-voltage measurements revealed details of the influence of π-conjugation on the single-molecule conductance.

Nano Letters, 2010

The conductance of a family of biphenyl-dithiol derivatives with conformationally fixed torsion a... more The conductance of a family of biphenyl-dithiol derivatives with conformationally fixed torsion angle was measured using the scanning tunneling microscopy (STM)-break-junction method. We found that it depends on the torsion angle between two phenyl rings; twisting the biphenyl system from flat ( ) 0°) to perpendicular ( ) 90°) decreased the conductance by a factor of 30. Detailed calculations of transport based on density functional theory and a two level model (TLM) support the experimentally obtained cos 2 correlation between the junction conductance G and the torsion angle . The TLM describes the pair of hybridizing highest occupied molecular orbital (HOMO) states on the phenyl rings and illustrates that the π-π coupling dominates the transport under "off-resonance" conditions where the HOMO levels are well separated from the Femi energy. * Towhomcorrespondenceshouldbeaddressed.

Nano Letters, 2012

Raman spectroscopy is able to probe disorder in graphene through defect-activated peaks. It is of... more Raman spectroscopy is able to probe disorder in graphene through defect-activated peaks. It is of great interest to link these features to the nature of disorder. Here we present a detailed analysis of the Raman spectra of graphene containing different type of defects. We found that the intensity ratio of the D and D' peak is maximum (∼13) for sp(3)-defects, it decreases for vacancy-like defects (∼7), and it reaches a minimum for boundaries in graphite (∼3.5). This makes Raman Spectroscopy a powerful tool to fully characterize graphene.

Macromolecules, 2012

ABSTRACT The supramolecular assembly of amphiphilic oligopyrenotide building blocks (covalently l... more ABSTRACT The supramolecular assembly of amphiphilic oligopyrenotide building blocks (covalently linked heptapyrene, Py7) is studied by atomic force microscopy (AFM) in combination with optical spectroscopy. The assembly process is triggered in a controlled manner by increasing the ionic strength of the aqueous oligomer solution. Cooperative noncovalent interactions between individual oligomeric units lead to the formation of DNA-like supramolecular polymers. We also show that the terminal attachment of a single cytidine nucleotide to the heptapyrenotide (Py7-C) changes the association process from a cooperative (nucleation–elongation) to a noncooperative (isodesmic) regime, suggesting a structure misfit between the cytidine and the pyrene units. We also demonstrate that AFM enables the identification and characterization of minute concentrations of the supramolecular products, which was not accessible by conventional optical spectroscopy.

Journal of the American Chemical Society, 2011

We present a combined experimental and theoretical study of the electronic transport through sing... more We present a combined experimental and theoretical study of the electronic transport through single-molecule junctions based on nitrile-terminated biphenyl derivatives. Using a scanning tunneling microscope-based break-junction technique, we show that the nitrile-terminated compounds give rise to well-defined peaks in the conductance histograms resulting from the high selectivity of the N-Au binding. Ab initio calculations have revealed that the transport takes place through the tail of the LUMO. Furthermore, we have found both theoretically and experimentally that the conductance of the molecular junctions is roughly proportional to the square of the cosine of the torsion angle between the two benzene rings of the biphenyl core, which demonstrates the robustness of this structure-conductance relationship.

Journal of Physics: Condensed Matter, 2012

We describe a new setup for simultaneous measurements of force and current in conductive nanocont... more We describe a new setup for simultaneous measurements of force and current in conductive nanocontacts in a liquid environment with a high sampling rate and resolution. A lab-built current-to-voltage converter allows measurements of the current over seven orders of magnitude. As examples, we studied conductances and mechanical forces upon formation and breaking of gold atomic contacts and of two molecular junctions containing 1,2-di(4-pyridyl)ethyne (M1) and 1,4-di(4-pyridyl)buta-1,3-diyne (M2). We found that the forces required to deform or break gold atomic contacts depend critically on the surrounding medium. Further, they show non-linear behaviour in dependence of the number N of gold atoms detached. The electromechanical properties of the two types of molecular junctions upon stretching were analysed by correlating breaking forces with simultaneously measured junction conductances. A rather complex behaviour in a wide range of forces was discovered. Comparison of the current-probe atomic force microscopy experiments on the rupture of molecular junctions with STM-based break junction experiments enables the assignment of breaking forces of molecular junctions to the corresponding junction conductances.

European Journal of Organic Chemistry, 2009

The design and synthesis of the macrocyclic turnstile 1 comprising a terminally sulfur-functional... more The design and synthesis of the macrocyclic turnstile 1 comprising a terminally sulfur-functionalized molecular rod and a redox-active catechol subunit is described. The shape-persistent macrocyclic scaffold consists of alternating arylene and ethynylene units. A freely rotating 2,6-diethynyl-catechol subunit is clamped between both terminal arylene subunits as molecular turnstile. While the electrochemical switching between the catechol and the quinone form of this catechol subunit is displayed by cyclic voltammetry, conformational rearrangements by favoring and disfavoring the formation of intramolecular hydrogen bonds are the subject of current investigations. Terminal acetyl-protected sulfur anchor groups enabled the immobilization of the macrocycle between an Au tip and an Au substrate of a STM set-up. Preliminary single-molecule transport investigations of the turnstile 1 display comparable values as for the parent molecular rod. An electrochemically-controlled single-molecule transport experiment to investigate redox-state-dependent transport properties is currently under way.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Dalton Transactions, 2014

Atomically thin layers of materials, which are just a few atoms in thickness, present an attracti... more Atomically thin layers of materials, which are just a few atoms in thickness, present an attractive option for future electronic devices. Herein we characterize, optically and electronically, atomically thin tungsten disulphide (WS 2 ), a layered semiconductor. We provide the distinctive Raman and photoluminescence signatures for single layers, and prepare field-effect transistors where atomically thin WS 2 serves as the conductive channel. The transistors present mobilities μ = 10 cm 2 V −1 s −1 and exhibit ON/OFF ratios exceeding 100 000. Our results show that WS 2 is an attractive option for applications in electronic and optoelectronic devices and pave the way for further studies in this two-dimensional material. Fig. 3 (a) Optical image of a WS 2 transistor. (b) Logarithmic conductance as a function of gate voltage with I on /I off = 100 000 at V b = 0.1 V. Inset: drain current I ds versus drain voltage V ds for various gate voltages.

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2007

Recently, we proposed a new four-parametric MGAB equation (modified Guggenheim-Anderson-De Boer e... more Recently, we proposed a new four-parametric MGAB equation (modified Guggenheim-Anderson-De Boer equation, GAB) for fitting of experimental isotherms of vapor-phase sorption on geosorbents. In the present study, the theoretical meaning of MGAB equation was elucidated. For this purpose, the properties of the equation of multilayer adsorption on a heterogeneous surface with local characteristics described by GAB model and Zeldovitsh-Roginskiy energy distribution function ρ(ε) were derived and the satisfaction of MGAB equation with these properties was shown. MGAB model has two parameters connected with GAB model: v m is the monolayer sorption capacity and k is the constant of the multilayer sorption, as well as two parameters of sorption energy distribution function ρ(ε): α, characterizing heterogeneity of ρ(ε) and C 0 , reflecting energy baseline. The influence of parameters α and C 0 on the shape of both ρ(ε) and model sorption isotherm was established. The applicability of MGAB model for the description of surface heterogeneity of a natural sorbent by example of 13 different sorbate-sorbent systems dried and moistened has been discussed. For these systems equilibrium sorption isotherms were measured, corresponding energy distribution functions ρ(ε) were calculated, and obtained α values were compared. On the whole, the differences in α values responded to the real tendencies of heterogeneity changes in these systems.

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2006

The approximation of isotherms of vapor-phase sorption of organic compounds (OC) is a long-standi... more The approximation of isotherms of vapor-phase sorption of organic compounds (OC) is a long-standing problem. It is especially difficult to solve in case of natural heterogeneous sorbents (soils, sediments, aquifers, etc.) since these sorbents contain sites with different sorption activity. The aim of this work was the statistical evaluation of the adequacy of the approximation of isotherms of vapor-phase sorption

CHIMIA International Journal for Chemistry, 2010