Facilities

We use various experimental techniques in our research. These are described below:


Scanning Tunnelling Microscopy

Basic Theory and Application

The scanning tunnelling microscopy (STM) relies on the quantum mechanical tunnelling of electrons, which enable electrons to traverse energy barriers which are classically forbidden. The rapid decay of the electronic wavefunctions with distance from a metal surface, as predicted through application of the Schrödinger equation, gives STM its high vertical sensitivity. Imaging, to atomic resolution, may be achieved by scanning the STM tip across the surface.

In STM, an atomically sharp metal tip (made of, for example, Au, Pt/Ir or W) is approached towards a sample until it is typically well within a nanometre of the surface. This gap acts as the barrier to electron transport. When a small bias between the tip and sample is applied, there is a finite probability that an electron from one electrode (i.e. the tip) can present itself at the other (i.e. the sample) as "leaked" tip and sample electronic wavefunctions overlap in the barrier.

STM can also be used to determine molecular conductance. For our conductance measurements, we use extremely sensitive electronics to measure the flow of current between the tip and sample, bridged by single molecules. If we know the applied bias and the current that flows through the molecule we can calculate the conductance, G, of the molecule using G = I/V. Further information about the specific experimental techniques we use at Liverpool can be found HERE. Examples of data and additional information on the statistical analysis we perform is detailed in the following reference:

The experimental determination of the conductance of single molecules
R. J. Nichols, W. Haiss, S. J. Higgins, E. Leary, S. Martin and D. Bethell
Physical Chemistry Chemical Physics, 2010, 12, 2801-15.

Our Equipment

We currently operate three Agilent scanning probe microscopes, an older 4500 and two 5500 systems (data sheet). All three are equipped for performing STM experiments in an electrochemical environment. We also have both 5500 systems fitted with environmental chambers in order to perform experiments under controlled conditions.

Images (Click the thumbnails for larger images)

We have our boxes for the 5500 system made in the department by our technician, Richie Dewson.



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Polarisation-Modulation Infrared Reflection Absorption Spectroscopy

Basic Theory and Application

Polarisation Modulation-Infrared Reflection Adsorption Spectroscopy, or PM-IRRAS, is a powerful surface characterisation technique with uses beyond determining the presence of a molecule on a surface. PM-IRRAS is particularly useful in determining the orientation of a molecule on the surface, and where monolayers, particularly those with a C-H backbone, are examined it can be used to determine the order of the monolayer both qualitatively1-4 and quantitatively.5, 6

PM-IRRAS is a form of infrared spectroscopy which it incredibly useful in the characterisation of molecularly functionalised surfaces. In PM-IRRAS the optical path of the incident IR beam passes through a grid polariser, which discriminates against s-polarised light, allowing only p-polarised light through. From here the p-polarised IR beam passes through the ZnSe optics of the Photoelastic Modulator (PEM), which rapidly switches the light between p- and s- polarisation through the expansion and contraction of the optical components using piezo crystals. From the PEM the IR beam hits the reflective substrate, upon which the molecule of interest is adsorbed, at typically close to grazing incidence (e.g. @ ~80° to the surface normal) and is reflected off at an angle equal to that of the angle of incidence, and finally passes into the IR detector. By hitting the substrate at near grazing incidence the maximum enhancement of electric fields near the surface occurs. This enhancement only occurs perpendicular to the plane of the metal surface, resulting in the "surface selection rule", which is key to the success of any IRRAS technique. The surface selection rule dictates that those dipole changes parallel to the surface will not be seen in the IR spectrum as they are cancelled by their image dipole, if, however, the dipole change is perpendicular to the substrate surface it will be enhanced by its oscillating image charge. With knowledge of the surface selection rule the intensity of certain bands within the spectrum can be used to determine the orientation of a molecule on the substrate. The PEM of the PM-IRRAS adds a layer of complexity to the experiment. As noted the PEM switches the incident IR beam between p-and s- polarised light, with the incident s-polarised light acting to discriminate against atmospheric gases as it is surface inactive, i.e. it cannot be used to detect dipole changes at the surface. Due to the polarisation modulation electro-optics, the PEM is optimised for a given wavenumber range. When the wavenumber strays from this optimised value the incident IR beam is composed of both p- and s- polarised light.7 As a consequence of this the wavenumber regions at which PM-IRRAS experiments are optimised and performed should be carefully chosen so as to optimise them to the regions of interest. This may mean that multiple experiments for various wavenumber regions may need to be performed for one system.

The result of a PM-IRRAS investigation is two set of reflectance signals (Rp+Rs) and (Rp-Rs) which can then be used to find the reflectance spectrum of the substrate.8 It should be noted that because of the polarisation modulation which occurs the resulting spectra has a baseline with the form of a Bessel function, and while the resulting bands can still be seen, baseline correction is conventional. The resulting spectra allow the main chemical groups of the monolayer to be identified, based on the wavenumber location of the resulting bands. The intensities of these bands can also be utilised to determine molecular orientation, and to compare the composition of related monolayer matrices.

Though performing a set of PM-IRRAS experiments can be fairly time consuming, it is well worth doing, as its pros far outweigh its cons. PM-IRRAS has all of the benefits of IRRAS, e.g. allowing determination of molecular orientation, while enhancing the system sensitivity, and scope. Furthermore the use of s-polarised light to remove atmospheric signals means that unlike in IRRAS a background scan of the system is not required.

PM-IRRAS has been employed by our group to characterise molecules adsorbed on surfaces, both under ambient conditions and electrolyte (in-situ PM-IRRAS).

  1. S. F. Bent, M. L. Schilling, W. L. Wilson, H. E. Katz and A. L. Harris, Chemistry of Materials,1994, 6, 122-126.
  2. H. Ron, H. Cohen, S. Matlis, M. Rappaport and I. Rubinstein, Journal of Physical Chemistry B, 1998, 102, 9861-9869.
  3. Y. T. Tao, Journal of the American Chemical Society, 1993, 115, 4350-4358.
  4. R. H. Terrill, T. A. Tanzer and P. W. Bohn, Langmuir, 1998, 14, 845-854.
  5. N. Battaglini, V. Repain, P. Lang, G. Horowitz and S. Rousset, Langmuir,2008, 24, 2042-2050.
  6. C. Nogues and P. Lang, Langmuir, 2007, 23, 8385-8391.
  7. R. C. Alkire, D. M. Kolb, J. Lipkowski and P. N. Ross, eds., Diffraction and Spectroscopic Methods in Electrochemistry, 1 edn., WILEY-VCH Verlag GmbH & Co., Weinheim Germany, 2006.
  8. T. Buffeteau, B. Desbat and J. M. Turlet, Applied Spectroscopy, 1991, 45, 380-389.

Our Equipment

We use the following equipment to perform PM-IRRAS experiments

  • Bruker IFS 66v/S Spectrophotometer with a Bruker PMA 37 module
  • Infrared Associates Inc. Mercury Cadmium Telluride (MCT) detector (model D313/6)
  • Hinds Instruments PEM-90 Photoelastic Modulator
  • Stanford Research Systems Lock-In Amplifier (LIA) (Model SR830)

Images (Click the thumbnails for larger images)

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Electrochemistry

Basic Theory and Application

Cyclic voltammetry is a technique used to investigate the electrochemical properties of a solution or a monolayer. It uses a potentiostat to sweep the potential up and down, while measuring the current. A 3-electrode set-up is used, with a working electrode, a counter electrode and a reference electrode.

Cyclic voltammetry potential waveform

The electrochemical reaction or process of interest takes place at working electrode. For surface electrochemical investigations monolayers may be adsorbed onto the working electrode surface. The counter electrode completes the circuit and it should have a large surface area so that it does not impede the rate of the reaction. Platinum mesh is often used due to its high stability in many electrolytes. The reference electrode is used to measure the potential of the working electrode. It should have a stable potential. Saturated calomel electrodes are widely used in aqueous electrolytes due to their stability and ease of use.

Cyclic voltammetry can be used to observe the redox properties of a compound in solution or as a monolayer and also to determine whether a reaction is reversible. Voltammograms can be used to determine the coverage of a monolayer, by integrating the peak to obtain the charge.


Example of a cyclic voltammogram of a monolayer of pyrrolo-tetrathiafulvalene on Au(111).

In the above example, the redox wave for an adsorbed monolayer of a pyrollo-tetrathiafulvalene PTTF derivative is observed. As the potential is swept positive the PTTF loses an electron to become PTTF•+. The potential range is limited by the surface oxidation of the Au(111) substrate in the aqueous electrolyte. As the potential is swept negative, the PTTF•+ is reduced and returns to PTTF.


Theory and Application of Cyclic Voltammetry for Measurement of Electrode Reaction Kinetics.
Nicholson, R.S.
Analytical Chemistry, 1965. 37(11): p. 1351-1355.

Structure-property relationships in redox-gated single molecule junctions - A comparison of pyrrolo-tetrathiafulvalene and viologen redox groups
E. Leary, S. J. Higgins, H. van Zalinge, W. Haiss, R. J. Nichols, S. Nygaard, J. O. Jeppesen and J. Ulstrup
Journal of the American Chemical Society, 2008, 130, 12204-+.

Our Equipment


Images (Click the thumbnails for larger images)


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Organic Synthesis

The majority of the molecules used for our measurements are made by the organic chemists within the group. Our synthesis laboratory is located in the Robert Robinson section of the chemistry department.

Click HERE for examples of recent synthesis.

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