(a) ACSTM system-level schematic. Two frequencies offset by a small difference Δf are combined, amplified as necessary, and connected to the antenna via a microwave coaxial cable. The loop antenna end is connected to the outer shield of the microwave cable, which is connected to a ground but isolated from the UHV chamber ground. The lock-in amplifier detects the response of the current at the difference frequency Δf and transmits the data (in polar format) to the control electronics. The electronics plot the z-position and the lock-in outputs at each point, thus producing a topography image as well as two spectroscopic images corresponding to the ∂C/∂V magnitude and phase, respectively. (b) Schematic of various scanning tunneling spectroscopies. The density of states is measured by modulating the bias voltage, V. The work function is measured by modulating the z-position of the tip, Z. The differential capacitance is measured by modulating the current directly using an applied microwave field.
Photographs of the antenna (a) outside the chamber, (b) inside the chamber but positioned away from the STM head to allow for sample exchange, and (c) encircling the STM tip during tunneling. In the RHK beetle STM design the tip is brought down from the top and rotated along the sample holder ramps until in position for tunneling.
(a) Difference frequency response of the ACSTM antenna at input frequencies from 80 to 1480 MHz in terms of difference frequency magnitude (top) and the difference frequency power in dbm (bottom). The antenna was tested using input frequencies offset by 5 kHz at a power of 10.1 and 10.5 dbm for the frequency and frequency +5 kHz signal, respectively. For example, the data point at 100 MHz above is thus extracted by measuring the 5 kHz signal based on input frequencies 100 and 100.005 MHz. In the power plot, each point is offset by 10.5 dbm to set the reference level to 0 dbm. The minimum loss is approximately −10.2 dbm at 308 + 308.005 MHz. (b) Simultaneously-acquired lock-in amplifier magnitude signals for the ACSTM ∂C/∂V and density of states ∂I/∂V response as a function of lateral antenna position. The antenna was moved using the linear translator from 11.2 cm to 11.5 cm, covering a the portion of the enclosed region in the loop that could be accessed without having the antenna come in contact with the STM tip or the piezeoelectric legs. While the ACSTM response is highly position-dependent, as is expected, the density of states signal is not. This indicates that the ACSTM response is unique from the differential current response. Two lock-in amplifiers were used with the same time constant and gain settings, and the input magnitudes for the microwave sources (ACSTM) and the bias voltage modulation (density of states) were set to approximately the same magnitude. The density of states curve was scaled by a factor of 1.3 to account for initial magnitude differences. All data for (a) and (b) were taken while in tunneling at parameters of −1 V and 10 pA.
Topography (a), (c), and ACSTM ∂C/∂V response (b), (d) of an n-type Si(100) sample with p-type dopant regions. At negative sample bias, the p-type region exhibits a depletion layer and thus exhibits a lower total capacitance, reflected in the lower contrast signal. Similarly, at positive sample bias, the n-type region exhibits electron depletion and thus appears with lower contrast than the p-type region. Topography (e), in-phase (f), and out-of-phase (g) images of the same sample at +1 V. All images taken at 5 pA setpoint.
Topography (a), (d), and ACSTM ∂C/∂V (b), (e), and (c), (f) dI/dV (DOS) images of P3HT films deposited on Au (a), (b), (c) and Pt (d), (e), (f). The simultaneously acquired ∂C/∂V images are shown at the same relative color scale. All images were acquired at −1.0 V sample bias, 10 pA tunneling current. The color bars are in units of nm for the topography images and pF/V for the ACSTM images. The dI/dV image color bar is in a.u. and thus merely shows low and high density of states. Compared to the ∂C/∂V images, there is no immediate correlation between high DOS regions and the carrier concentration. The lower ∂C/∂V signal in (e) indicates an excess of mobile holes for the P3HT/Pt case relative to the P3HT/Au case in (b).
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