Laser Driven Scanning Tunnelling Microscopy
6.2.2 Specific Procedures Used
The procedure used to conduct LDSTM experiments is as follows:
The laser is configured to emit the required idler wavelength and is directed at the surface near the tip. The tip is approached towards the surface until the required
147 tunnelling current is detected. The surface is then imaged and the tip is moved across the surface until a suitable part of the surface is found. This usually is an area (500nm)² where the vertical variation is less than 2nm. Ideally within this area there would be a section of Au(111) terrace that is at least (100nm)² in size. The size of the area imaged is repeatedly reduced to about 50% of its previous size until the image is (20 → 50nm)² in size. Then the tip is positioned in the centre or bottom left corner of this area, in the STM’s ‘standby’ mode.
At this point if required the power of the idler beam can be reduced by means of an iris in an attempt to reduce the effect of the energy of the laser heating up the surface such that the adsorbed molecules are evaporated from the surface.
The tunnelling parameters (V, I, feedback loop) are adjusted to increase the size of the response to the laser. The feedback loop is usually set to 3% (see below). The tunnelling current is set to –4nA to increase the signal to noise ratio of the current modulation. The voltage was increased to 0.4V to compensate for the decrease in the tip-sample separation due to the increase in tunnelling current. The position of the beam is then adjusted to maximise the modulation of the tunnelling current. The lock-in amplifier settings are optimised for the signal measured. An HP-VEE program then records a batch of readings from the lock-in.
One of the experimental variables is changed, such as the AOM frequency or the laser wavelength and another batch of readings is recorded. This process is continued until enough batches have been collected for the variable used.
Of the nine parameters that can be adjusted, (listed as (a) to (i) above) experiments were only conducted to see how five of them affected the modulation of the tunnelling current. In principle, the feedback setting could affect the lock-in response but for the purposes of this study it was fixed at 3%. This is a relatively low setting within the typical range of the feedback loop. Most of the changes in the tunnelling current are not compensated for and passed through to the lock-in amplifier but the tip is moved vertically in response to low frequency changes in order to stop the tip colliding into the surface.
The power of the laser beam before the final lens was fixed at 10mW. Some initial experiments were conducted with the power greater than this but as the size of the beam on the sample was reduced with modifications to the arrangement of the optics there was some concern that the laser power was enough to remove some of
Scanning Tunnelling Microscopy
148 the thiol molecules. It was occasionally observed that STM images of areas of the sample exposed to the laser beam for ≈10 hours showed exposed areas of the Au(111) substrate. Consequently the laser power was reduced to a lower value of 10mW. Again as with the feedback setting it was expected that varying the laser power would affect the lock-in response but understanding the effect of the other variables was considered more important.
The lock-in time constant was fixed at 1s as a compromise between being long enough to average out the fluctuations and short enough to be able to make changes (for example to the beam position) and see the response almost immediately. In addition to the averaging performed by the lock-in, an average is taken of the batch of data recorded from it.
As far as the positioning of the tip on the sample is concerned, it is assumed that there is no special significance of any particular place on the sample chosen. The only criterion for a suitable location is the quality of the underlying gold substrate. This leaves the five parameters that are used as independent variables.
6.2.2.1
Laser Modulation Frequency
Some experiments were conducted where the modulation frequency of the laser is varied from 316 Hz to 100 kHz. It was expected that the response of the STM to the modulation frequency would be similar to that seen in previous LDSTM studies[4, 5] (see §2.5) but it was not clear how having a different laser wavelength and spot size would affect this. It was expected that at low frequencies thermal effects would dominate the LDSTM response but that as the modulation frequency was increased these would roll-off and any other effects would become visible. The experiments were conducted to investigate this and their data is discussed in §6.3.1.
6.2.2.2
STM Electronics
In addition to modulating the tip sample distance with the laser, a batch of data was recorded where the current was varied by varying the applied tunnelling
149 voltage in a similar way to the STS experiments. Although unlike the STS experiments, the modulation in the applied voltage is provided by an external signal generator. The compensation box is only able to modulate the tunnelling voltage up to a frequency of ≈ 8 kHz so this alternative facility was required to explore the frequency response of the STM electronics. The results from this experiment are discussed in §6.3.2.
6.2.2.3
Tunnelling Current
A series of experiments was conducted recording the effect of the equilibrium tunnelling current on the modulation of the tunnelling current. The tunnelling current was varied from 0.25nA to 5nA for a range of laser modulation frequencies from 316 Hz to 10 kHz. From §2.5.1, Equation 2.54 states that:
cI dz dI
−
= Equation 6.1
Given that c is positive, this equation predicts that dIdz will be negative for positive values of the tunnelling current. Although for the LDSTM experiments the lock-in amplifier measures the change in I with respect to the changes in z, it measures this as a vector, either with x and y components or as R and θ. However, the quantity dzdI is a scalar but it can be related to R and θ by:
dz dI
= R Equation 6.2
When the lock-in response is plotted against a chosen variable, its magnitude R is used so from these relations R would be expected to be proportional to I with a positive constant of proportionality. The results from experiments investigating the effect of the equilibrium tunnelling current are discussed in §6.3.3.
Scanning Tunnelling Microscopy
150
6.2.2.4
Applied Tunnelling Voltage
For most of the LDSTM experiments the tunnelling voltage chosen was 0.5V. Electrons at this potential have enough energy to excite an inelastic channel at ≈ 0.36eV due to a C-H stretching mode in the thiol molecules on the sample. If the LDSTM response involves an inelastic process then it might be expected that if the tunnelling voltage was varied to include energies lower than 0.36eV then a change in the LDSTM response might be observed. Consequently a batch of experiments was run in which the tunnelling voltage was varied from 0.1V to 0.6V to investigate this and the results are discussed in §6.3.1.
6.2.2.5
Laser Wavelength
Another possibility is that processes such as inelastic tunnelling might also have some dependence upon the wavelength of the laser used. To explore this, a batch of experiments was run initially across three laser wavelengths corresponding to the 29.5µm, 29.75µm and 30µm gratings at 100ºC. The results are discussed in §6.3.5.
6.2.2.6
Chemical Nature of the Sample
In addition, for each of these variables, experiments were conducted on gold samples both coated with methylthiolate from DMDS and uncoated to look for differences due to the thiolate adsorbate. The results are discussed §6.3.4.
6.2.2.7
Surface Topography
Finally, some experiments were conducted where the LDSTM lock-in response was measured whilst the tip was scanned across the surface. The tunnelling current was reduced to 1nA to minimise the effect of the tip dragging any
151 adsorbed molecules across the surface. No correlation could be seen between the output of the lock-in and the surface topography.
6.3 Results and Discussion
Experiments were conducted to study the effects of five variables; the laser modulation frequency, the applied voltage, the tunnelling current used, the laser wavelength used and the sample type. The details of the experiments conducted and the results obtained are given below.
6.3.1 Laser Modulation Frequency and Applied Tunnelling