Light Source and Calibration

For photodependent measurements a ThorLabs TLDC4100 was used, with LEDs of 455, 530, 590 and 625 nm wavelengths, chosen to match the absorption maxima of P3HT:PCBM [208]. This was connected directly to the MFP3D base, replacing the existing light source. This allowed illumination from above or below the sample, using the standard optics in the MFP3D dual view base.

The ThorLabs light had a few features that made it a good choice for this project. Firstly, the only adjustments needed to connect it to the existing light path were two relatively cheap adaptors. Once in place it readily attached to the existing light guide. Secondly, the controller for the lighting can be driven by IgorPro which also controls the AFM, so the operation of the light and the AFM can be simultaneous. Thirdly, each of the four LEDs can be modulated individually up to

1 MHz.

To calibrate the light output, a Vishay BPW21R silicon PN photodiode was first calibrated using an external quantum efficiency (EQE) setup, as discussed in [209] and shown schematically in Figure 2.15. A Newport Oriel solar simulator pro-

Figure 2.15: A schematic overview of the EQE system used for photodiode calibration

vides the incident light, which is then monochromated using an array of diffraction gratings, the first allows the continual changing of wavelength and the later gratings are used selectively to block lower order wavelengths when longer wavelengths are measured. The beam is chopped at 50 Hz allowing a Stanford research SR 830 lock in amplifier to be used, increasing the accuracy of measurement. The light is then focussed onto an approx. 50% beam splitter, whereupon two mirrors feed the light to a reference diode (DR) and a calibration diode (DC). A ratio is taken between the responses of DC and DR. For every mA measured on the calibration diode, the response on the reference diode is known. Then when a sample, in this case the photodiode, is tested the current measured is multiplied by the ratio between the two diodes to achieve the corrected response.

The EQE itself relates the number of photons converted into electrons at a specified wavelength incident on the active area of the device. From this a spectral responsivity can be calculated, that is the amount of electrical output to optical input. The responsivity is calculated as

R(λ) =EQEqλ

hc (2.6)

whereλis the wavelength of the incident photon, h is Planck’s constant and c the speed of light. The measured spectral responsivity of the photodiode used is shown

in Figure 2.16.

Figure 2.16: Spectral responsivity of a silicon PN photodiode.

Next, the spectral power of the light getting to the photodiode is needed. The transmittance of optics in the light path and the relative spectral radiance of the light are needed for this. This data can be seen in Figure 2.17.

Figure 2.17: a) Relative power output spectra of light from the ThorLabs website [210]. b) Transmittance of the light guide at- tached to the AFM, from the Dolan-Jenner website [211].

Whilst the spectra of Figure 2.17 a) are most likely to be accurate, the intensity is relative and hence needs measuring, for this purpose, the calibrated

photodiode was employed (calibration as described earlier). By aligning the photo- diode in the AFM light path, the current produced by each light over the 7.5 mm2 of the photodiode can be measured. The results of this are shown in Table 2.1. This was performed with the light completely covering the photodiode, and so does not show the power of illumination going through the AFM optics, but instead shows the spectral intensity. Table 2.2 shows a comparison of the measured power at each

LED Wavelength(nm) Current on Photodiode (mA) Power (W/m2)

Red 625 0.182 83.74

Blue 455 0.093 52.24

Yellow 590 0.025 10.54

Green 530 0.054 23.23

Table 2.1: Current response of calibrated photodiode with incident light from ThorLabs 4LED light source and their calibrated power.

stage of the optical path from the light to the sample stage in the AFM. These numbers indicate that less than 1% of the light output actually reaches the sample. Approximately 50% of the light is lost in the light guide, the rest is lost in the AFM base itself. Focussing the light onto the photodiode with a small aperture, thereby

Position Area (m2) Current on Photodiode (mA) Power (W/m2) after fibre

7.5× 10−6 30 14000±1000

before AFM

after AFM 7.5× 10−6 0.354 170±20

after AFM 4.1× 10−8 0.006 530±50

Table 2.2: Current response of calibrated photodiode with incident light from ThorLabs 4LED light source, and their calibrated power.

ensuring that all the light is hitting the photodiode across an area of 4.1 × 10−8 m2 yields the total flux hitting the photodiode to be 525 W/m2. Hence it can be assumed that 525 W/m2 of light is incident on each sample with the illumination at full power. These calculations used the bottom optical path of the AFM, the top path is more difficult to measure as the photodiode cannot be in place while the AFM head is, however, this will be discussed later in Chapter 4.3.4.

When applying illumination through the bottom optical path of the AFM it is worth considering the possible enhancement of the electric field around the tip by surface plasmon resonance. It is unlikely that this will affect the top optical path as the tip itself will be shadowed by the cantilever. It is also unlikely that surface plasmon resonance will have a significant effect in this system as the expected

wavelength for plasmon resonance of platinum with a diameter<100 nm is around 295 nm [212], well into the UV spectrum.

2.2.8.1 Light Control and Tr-EFM Process

The light controller itself is connected to the computer by USB and the controller can be sent information by VISA commands. These commands can fully control the light sources, however, they incur a significant signal delay and cannot be used to send complex signals to the light source. Since for Tr-EFM the light pulses need to be sent at specific times, the Macrobuilder in IgorPro is used to control the lights. Appendix 5.3.3 shows how this is achieved.

2.2.8.2 Analysis - TrEFM

The analysis of the Tr-EFM data is somewhat similar to the analysis of the force volume maps, but in practice much simpler. Firstly the relevant waves are loaded into memory. Then the light oscillation wave is used to find the total length of an oscillation (of the light turning on and off). The program then converts the 1D wave into a matrix format where each row of the matrix corresponds to a single oscillation. From there an average across the rows is calculated.

This average wave is fitted from the point where the phase rises or falls by 10% after the light has been applied until 10 points before the light is turned off, if measuringτc. Forτd, the fitting starts 10 points after the light turns off and is fitted until the end of the data, which is when the bias is turned off. This method ensures that the fitting does not get skewed by random noise in the data. The relevant parameters, namely the measuredτ, can be extracted.

2.2.8.3 Signal Modulation and Speed

Tr-EFM is comparatively easy to implement, requiring no external equipment other than the AFM and light source. However, the speed of the electronics in the AFM introduces a limit to the time resolution.

Figure 2.18 shows the time resolution of the DAC from the AFM controller. Here, a programmed square wave is sent through the controller to an oscilloscope. The scope shows that there is a significant rise time of the signal being sent by the DAC. In order to eliminate this delay, the controlling signals can be sent via an external signal generator, as required for IM-KPFM.

Figure 2.18: Square wave signal sent by the AFM in blue, and the actual signal as recorded by an oscilloscope.