Torsionally Stabilized Nano Impedance Microscopy


in impedance is then entirely attributed to the molecule.


Materials and Methods For TR-NIM experiments,


Figure 1: Torsional resonance nanoimpedance microscopy (TR-NIM) schematic diagram (a). An AC bias is applied to the tip/sample, and the alteration in this signal due to the impedance of the sample is output to the amplifier. The signal from the amplifier is summed with the compensating signal to account for the capacitance of the entire system. This combined signal then enters the lock-in amplifier. The output of the lock-in is fed into the microscope controller for analysis. (b) Schematic illustration of the tip-surface junction allowing photo excitation of patterned protein layers. (b) is from Kathan-Galipeau et al., ACS Nano 4835, copyright 2011, with permission of ACS.


the system and the impedance of the sample. Given the film size and properties, a significant contribution to the signal is noise from the system’s capacitive coupling. Tere is capacitive coupling related to the cantilever, cantilever holder, and other structural components of the AFM. Terefore, the nulling signal is used to account for most of the effect of these other signals. It is assumed that the impedances of the graphite and sample are in parallel. Assuming the molecule can be represented as an RC


circuit [20], R =


102VG sX


and C = sY 102VGω (1, 2)


where R = resistance, C = capacitance, V = applied voltage, ω = frequency, s = sensitivity of lock-in amplifier, G = gain of current amplifier, and X and Y = output of lock-in amplifier. (2 is required because Vo is an RMS voltage; 10 is a conversion factor within the lock-in amplifier.) Te values for X and Y are relative to that of the graphite. If the compensating signal is perfect, we measure zero impedance over graphite. Te change


a platinum-iridium coated tip was used; fo = 146–236 kHz and k = 21–98 N/m (Nano- sensors). Te AFM instrumen- tation was performed on a Veeco Dimension 3100 with a IVa controller. Two current amplifiers were used, which together provide a gain range of 1 × 102 – 1 × 1011 V/A (Femto DLPCA-200, DHPCA-100) Two lock-in amplifiers provided a frequency range from 1 mHz–200 MHz (SRS830,


SRS844). Te AC signal can be applied in a range from 1 µHz to 30 MHz (SRS DS345). Te sum box has a range from 0.002–60 MHz (Mini-Circuits ZSC-2-2).


TR-NIM on Hard Materials To demonstrate the effectiveness of TR-NIM for probing


dielectric behavior, Figure 2 shows a typical TR-NIM image of CdTe. Figure 2 shows the topography, amplitude, and phase of impedance (leſt to right). At 70 kHz, the resistance is 8035 kΩ, and the capacitance is 21.3 fF. A dielectric con- stant of 10.1 was determined from these images, assuming 1 mm thick CdTe and a 8.7 nm tip size. In comparison, the literature value of the dielectric constant of CdTe is 10.9 [21]. Tis agreement provides confirmation of the effectiveness of TR-NIM for determining the dielectric constant of materials.


TR-NIM on Soft Materials: Biomolecules Toward Protein-Based Circuits


Recently, the appreciation of the high energy yield and


quantum efficiencies of a number of natural photo-activated proteins, in the context of a decade of research on molecular electronics, has raised the possibility of new nanoelectronic


Figure 2: TR-NIM image of CdTe ⟨0001⟩ topography, amplitude, and phase of impedance (left to right). Height z-range = 15 nm, amplitude range = 80 mV, phase range = 120 mV. Parameters: 70 kHz, 0.5 V applied signal, gain = 109, sensitivity = 5 mV. Platinum-iridium coated cantilevers were used with the following properties: resonant frequency = 146–236 kHz, spring constant = 21–98 N/m, and tip radius ~ 40 nm.


18 www.microscopy-today.com • 2011 November


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76