Preliminary work for Local density of states measurements using STM/STS techniques on Quazar Technologies' nanoREV STM

1. Local density of states measurements using STM/STS techniques
Chintalagiri Shashank∗
Department of Physics, Indian Institute of Technology Kanpur, Kanpur 208016
(Supervisor : Prof. Deshdeep Sahdev)†
(Dated: April 20, 2012)
The goal of the project is to develop the infrastructure and expertise necessary for Scanning Tun-
neling Spectroscopy (STS). Taking a two pronged approach, we have moved closer to this goal and
qualitatively demonstrated the ability to perform STS on various samples. The principle infrastruc-
ture necessary for this activity, specifically a lock-in amplifier, and its integration with an STM not
designed for STS is described. Various sets of data acquired, indicators of the fidelity of the data,
and the rationale for the designs chosen are also presented.
PACS numbers: 07.79.Cz, 68.37.Ef
I. INTRODUCTION
The Scanning Tunneling Microscope (STM) was in-
vented in 1981 by Gerd Binnig and Heinrich Rohre at
IBM Zurich. An STM is a non-optical microscope which
typically has atomic resolution. The STM is based on the
concept of quantum tunneling. When a conducting tip
is brought within a few Angstroms of the surface to be
examined, a bias voltage applied between the tip and the
sample can allow electrons to tunnel through the vacuum
or air barrier between them. The resulting tunneling cur-
rent is a function of tip position, applied voltage, and the
local density of states (LDOS) of the sample. Informa-
tion is acquired by monitoring the current as the tip’s
position scans across the surface. The use of an STM is
generally limited to electrically conducting samples, and
requires extremely clean and stable surfaces, sharp tips,
excellent vibration isolation, and sophisticated electron-
ics.
Since its invention, a number of applications have been
found for the STM and many variants developed to per-
form measurements of different kinds. The application
targeted in this project is that of Scanning Tunneling
Spectroscopy (STS). In order to achieve this goal, an ad-
ditional step of the development of a lock-in amplifier
was identified. Section III deals with this effort. Simul-
taneously, the rest of the steps necessary for STS were
performed using a commercial lock in amplifier, the de-
tails of which are included in section II.
II. SCANNING TUNNELING SPECTROSCOPY
Scanning Tunneling Spectroscopy (STS) is a technique
used to characterize electronic distribution as a func-
tion of energy at the surface of conducting and semi-
conducting samples with the help of an STM1–3
. It can
be used to measure the local density of states at various
points on the surface and generate conductance maps of
the surface. Band gaps can also be measured using this
technique. It can be shown that the local density of states
is well represented by the differential tunneling conduc-
tance dI/dV, where I is the tunneling current and V the
bias voltage.
The focus in this project is to demonstrate STS by
measuring the local density of states of three samples,
namely Gold (Au), Highly ordered pyrolytic graphite
(HOPG), and Bismuth Telluride(Bi2Te3). These sam-
ples are well documented in literature, such as in4,5
.
A. Experimental Setup
The experimental setup is centered on the nanoREV
Air STM. A lock-in amplifier (LIA) is used to measure
tunneling conductance directly. The data presented here
was taken using a commercially available EG&G Prince-
ton Applied Research Model 5209 lock-in amplifier. In
addition, a Tektronix TDS210 digital oscilloscope was
used for testing the setup and monitoring various phases
of the acquisition, most commonly the amplified tunnel-
ing current and the output of the lock-in amplifier. Fig-
ure 1 shows a simplified block-diagram of the experimen-
tal setup.
The nanoREV Air STM consists of two distinct sec-
tions. The first is the scan head, which contains the
sample holder, tip, piezos, and the Tunneling Current
Amplifier (TCA). The scan head is enclosed inside a
shroud and is placed on a vibration isolation platform.
A spring-mass system is used to damp external vibra-
tions and isolate the scan head from them. Additionally,
a cover provides shielding from ambient air flows. The
second part of the nanoREV STM is the electronics box,
which contains the bulk of the electronics of the STM.
The electronics box contains the circuitry necessary to
generate a DC bias voltage, which can be applied to the
sample holder, drives the piezoelectric actuators. The
feedback and acquisition circuitry is also located inside
the electronics box.
The various constraints marked in the figure are those
of bandwidth and frequency limitations of the smaller
blocks. These are discussed below, with their effective
values in the unmodified STM and the implications in
terms of the experimental design.

2. 2
FIG. 1. Simplified block diagram of STS experimental setup,
showing only the relevant blocks. Blocks in blue are additions
to the regular STM setup. The relevant constraints in band-
width and timing are marked in the diagram and discussed
in the text. The details of the lock-in amplifier are dealt with
separately.
BWTCA: Bandwidth of the TCA. The TCA, operating
at a gain of 109
, has a bandwidth specified by the
RC constant of the circuit at its input, along with
the pin, of about 800 Hz. This bandwidth was also
observed experimentally in the form of attenuation
in the AC modulation of the signal beyond 1 KHz.
The high gain in the TCA is necessary to amplify
the tunneling current from the picoampere range
to a voltage level which is easier to handle.
BWIA: Bandwidth of the Instrumentation Amplifier that
follows the TCA. The instrumentation amplifier is
operated at a gain of 1, and its purpose is to convert
the differential output signal of the TCA into a sin-
gle ended signal referred to the circuit ground. This
signal is then used in the feedback circuit as well as
can be digitized. This amplifier has a bandwidth of
1 MHz at a gain of 1. The combined bandwidth of
the TCA and the IA, well represented by the lower
of the two values, presents the effective limitation
on the AC modulation of the bias voltage that may
be applied for STS measurements.
fFB: The feedback circuit of the STM is used in the con-
stant current mode, where the Z-direction piezo is
used to maintain the tip at a height where the tun-
neling current is kept constant. When scanning,
this makes the tip roughly follow the topology of
the surface. When an AC modulation is applied
over the bias, it is necessary to ensure that fre-
quency of the modulation is higher than the fre-
quency response of the feedback, so that the feed-
back does not cause additional movement of the tip
at that frequency. The frequency response of the
feedback was estimated to be around 600 Hz.
fREF: The frequency reference signal generated by the
lock-in, and therefore that of the AC modulation
of the bias. This frequency is to be chosen so as to
remain within the bandwidth of the amplifiers, and
above the response frequency of the feedback.
fSample: The effective sampling rate is dependent on the
mode the STM is used in. For constant height
imaging and spectroscopy, where the feedback is
turned off during the actual acquisition of data, the
effective sampling rate needs to be high to suppress
the effects of thermal drift in the piezos, as well as
creep that may arise. The upper limit on this fre-
quency is determined by the time it takes for the
repositioning of the tip in the case of scanning, and
the time it takes to acquire the data from the lock-
in, in the case of spectroscopy.
tWait: There are a few cases in which a wait time be-
tween samples is required. In the case of imaging,
it is the time for the tip to relocate and the feed-
back to correct the Z-position of the tip. In the
case of spectroscopy, two wait times are necessary.
The first is a wait time between sweeps of the bias
voltage, during which the feedback may be allowed
to correct any drift that may have occurred dur-
ing the sweep. Additionally, a wait time between
samples is necessary to allow the lock-in output to
stabilize.
The lock-in amplifier adds a few constraints in terms of
the time it takes for its output to be valid. This is a con-
sequence of way in which the lock-in amplifier functions,
and the final stage of the lock-in amplifier being a low-
pass filter. For the STS measurements taken, we assume
that the lock-in output is valid after approximately ten
time periods of the reference signal. The time constant
of the low pass filter is selected to be around three time
periods of the reference signal.
B. STM Modifications
A few changes were necessary to allow the STM to per-
form STS measurements. The changes and the problems
they were designed to overcome are described below.
1. Addition of AC modulation
The modulation of the bias voltage is necessary to be
able to use the lock-in amplifier. The intended conse-
quence of using an AC modulated bias instead of a DC
bias or a simple sweeping of the bias across the range
of interest is to suppress 1/f noise in the measurement
and to accurately measure the differential conductance
(dI/dV), instead of attempting to calculate it numeri-
cally from noisy data. An adder circuit was added in the
bias generation path, so as to be able to inject the AC
reference signal into the bias.
Further, It was found that when the AC modulation
was applied, it was being picked up by the TCA. Since
the goal is to measure the effect of the change of bias on
the tunneling current, this pickup is detrimental to the
measurement. Various experiments performed on this

3. 3
(a) Au Large Area Scan (b) Bi2Te3 Large Area Scan (c) HOPG Large Area Scan
(d) Au Small Area Scan (e) Bi2Te3 Small Area Scan (f) HOPG Small Area Scan
(g) Au Conductance Map (h) Bi2Te3 Conductance Map (i) HOPG Conductance Map
FIG. 2. Images taken with the modified nanoREV 4.0 Air STM
pickup by varying the amplitude resulted in the following
observations :
1. The pick-up existed even in the absence of a tunnel-
ing current. Increased modulation amplitude led to
increased pick-up amplitude.
2. The frequency of the pick-up matched the fre-
quency of the modulation. The two signals were,
however, not in phase. The phase relationship was
found to be frequency dependent. At a 1 KHz mod-
ulation frequency, the phase difference was about
20 degrees.
3. The amplitude of the pick-up increased with in-
creasing modulation frequency up to about 1 KHz.
It then stabilized up to around 3 KHz, after which
it started reducing slowly. The pickup survived up
to a modulation frequency of 50 KHz.
This suggested that the pick-up may have been partially
capacitive, and that the TCA bandwidth limitation was
appearing in these experiments as well. In order to mini-
mize the pick-up, a metallic shield was added between the
tip and the sample, with a small slot provided through
which the tip could reach the sample for tunneling. This
shield reduced the pick-up level by an factor of three.
2. Changes for digitization of differential conductance
The output of the lock-in amplifier, being the differ-
ential conductance (dI/dV), needed to be digitized and
stored. Even though the lock-in amplifier used allowed
for computer access via GPIB, it was decided not to use it
so as to minimize the changes necessary when the home-
made lock-in amplifier was swapped in. An analog mul-
tiplexer at the input of the ADC was added, so as to
digitize the lock-in output instead of the tunneling cur-
rent when making STS measurements. For better LDOS
measurements, though, both will have to be measured
simultaneously so that (dI/dV / I/V) can be calculated,
which is closer to the local density of states.
In addition, the necessary changes in the software to
adjust the acquisition rate when obtaining spectra or con-
ductance maps such that the temporal spacing between
adjacent readings was longer than 10 times the time pe-
riod of the reference signal were made.
Further, it was noticed that the slower acquisition of
the spectra, and the increase in the number of sweeps
taken to 30, introduced significant errors in the spectra.
It was determined that the cause of these errors in the
spectra were due to thermal drifts and creep in the piezos,
which cause a slow change in the tip-sample separation.
In order to compensate for this, an additional wait state
was added between two adjacent sweeps, with the feed-
back turned on. This allowed the feedback to correct for
any drift that may have taken place during the sweep.

4. 4
3. TCA bandwidth enhancement
The bandwidth limitation of the TCA was discussed
along with the other timing constraints mentioned pre-
viously. It was desirable to have a higher modulation
frequency, so as to decrease the acquisition time and
the associated drift during a single sweep. Addition-
ally, the frequency response of the feedback was close to
the allowed modulation frequency and would have inter-
fered with conductance map imaging. To suppress these
sources of error, the bandwidth of the TCA was increased
from 800 Hz to approximately 8 KHz by redistributing
the gain between the TCA and the following instrumen-
tation amplifier. By reducing the amplification at the
TCA state to 108
from the original 109
, and adding a
gain of 10 at the instrumentation amplifier, the effective
bandwidth (which was dictated solely by the TCA) was
increased by a factor of 10.
4. Validation of changes
In order to validate the STM itself after these changes,
a number of images of the three samples were taken, some
of which are shown in figure 2. The top row of images
are large area scans showing the surface topology. The
middle row shows the topology of a smaller area, and the
bottom most row shows the conductance map obtained
for that area. Of these small area scans, the ones of
HOPG are of atomic resolution.The conductance maps
shown in the bottom row of images were obtained using
the commercial lock-in amplifier by reducing the acquisi-
tion time during imaging to its lowest setting of 1.5 usec.
Slower acquisition will likely be required to get cleaner
conductance maps.
In addition, a few images from an attempt to obtain
atomic resolution images with Bi2Te3 with AC modula-
tion of the bias turned on are shown in figure 3. This was
done using the modified STM. While hints of the atoms
were visible, clear images at atomic resolution were not
obtained. Further refinement of the process, including
preparation of the tip and the surface just before imag-
ing, may be necessary before success can be declared in
this direction.
C. Methods
1. Topographical Imaging
The process of topographical imaging using an STM
is well established in literature and in practice. There
are two distinct modes of imaging - constant height and
constant current. Constant height imaging involves open
loop control of the STM tip, moving it over the surface
at a fixed z position. This form of imaging is susceptible
to thermal drift and comes with the possibility of the tip
colliding with features in a rough surface, but provides
(a) Bi2Te3 large area topology
(b) Bi2Te3 scan with discernible atoms
FIG. 3. Topographical Scans of Bi2Te3
for a greater resolution. The image of the surface itself is
generated using the tunneling current measured at each
point. None of the images presented here are constant
height images. Typically, constant height imaging is pre-
ferred when attempting atomic resolution scans. It is
possible that performing these scans using the constant
height mode will produce cleaner images.
The second mode, constant current imaging, is used
when imaging a large area or using rough surfaces. In
this case, the tip’s z position is maintained via closed
loop control, where the tip is moved to maintain a fixed
tunneling current. The image is generated from the tip’s
z-position at each point which produced the specified tun-
neling current. The images presented here have all been
taken in the constant current mode. This allowed us
the advantage of being able to acquire these scans at a
slightly lower rate, thereby producing topographical im-
ages with closer parity to the conductance maps.
The process of topographical imaging starts with at-
tempting to correct for any local slope that may exist in
the sample in the region of interest. The X-slope and Y-
slope can be modified separately to allow the electronics
to compensate for such features. And other long range
slope that is observed in the image, as it often is, is due
to to the thermal drift of the piezo during the course of
the imaging. The correction for this drift is incorrectly
marked as a slope in the surface. It is compensated for
finally during the course of processing the image.
For each scan, the image is acquired row-by-row or
column-by-column. Each row (or column) is acquired
twice, once in the scan direction and again in the retrace
direction, where the tip moves back along the path it
traced. Positive correlation between the scan and retrace
image confirm that the data acquired corresponds to real
features in the surface and is not the effect of a transient
disturbances. All the scans presented here are shown

5. 5
along with the retrace images acquired.
The scans shown in figures 2a, 2b, 2c, 2d, 2e, 2f, 3, and
5a are all topographical images.
2. Acquisition of Spectra
The acquisition of conductance spectra as well as I-V
curves are done at a specified point. The steps involved
in topological imaging are noted below :
1. A topographical image of the surface is first ac-
quired, and a point of interest is marked.
2. The tip is moved to that location, and is set to
constant current mode at a nominal bias voltage
for an initial settling time. This brings the tip to
a reasonably well defined point with respect to the
sample.
3. The feedback loop is stopped and spectroscopy be-
gins. The bias is set at the lowest point in the range
of interest, and the tunneling current in the case of
I-V spectroscopy or the lock-in output in the case of
differential conductance spectroscopy are digitized
after a duration specified by the sample wait time.
This wait time is determined by the frequency of
the modulation and other parameters, as discussed
previously.
4. The bias is moved up by the defined step size, and
the process is repeated until the maximum bias
voltage of interest is reached.
5. The bias is set to the nominal point and feedback
is turned on. A 5 second wait time allows the tip
to return to the point it started at, if there was any
drift during the intervening time.
6. The process is repeated for as many sweeps as re-
quired.
The conductance spectra shown here were obtained by
taking 30 sweeps one after the other using the process
defined above and the results averaged. The other pa-
rameters involved are :
Modulation Frequency: 2.5 KHz
Modulation Amplitude: 30 mV
Lock-in Time Constant: 3 msec
Acquisition Time: 5 msec
Nominal Bias: 200 mV
The spread of data obtained in a single set of sweeps is
believed to be caused by an as yet uncorrected thermal
drift in the piezo. Methods to further enhance acquisition
rate, given the various bandwidth constraints, are being
explored.
3. Conductance Imaging
The conductance maps presented here, in figures 2g,
2h, 2i, and 5b have been obtained using the same process
used to acquire the topological images. And additional
digitization mode was added to the software, which would
use the process used for constant current imaging to ac-
quire the data. A longer acquisition time was used to
acquire the images due to the low modulation frequency,
as discussed above. Figure 5 shows a conductance image
acquired of Bi2Te3 and the corresponding topological im-
age of the same region.
III. LOCK-IN AMPLIFIER
The measurement of small signals is a subject of great
interest in very many fields of experimental science, in-
cluding physics, chemistry, biology, and the many inter-
disciplinary fields that have arisen. The need for such
measurement stems from the fact that a number of phe-
nomena are characterized by small but ultimately de-
tectable changes in a measurable parameter. Conven-
tional measurement techniques, however, fail to measure
the small changes accurately due to a number of reasons.
The use of a lock-in amplifier enables the measurement
of signals with amplitude much lower than the noise of
the measurement itself.
A. Motivation
Lock-ins are used in a wide variety of applications. The
basic requirement for a measurement to be compatible
with a lock-in is that the physical phenomenon to be de-
tected should be such that it can be turned on and off,
or modulated, according to an external signal called the
’reference signal’. The frequency of this reference signal
is constrained by the capabilities of the lock-in ampli-
fier, other instruments in the experimental setup, and
the characteristics of the physical phenomena involved.
In the context of this project, the lock-in amplifier was
looked at to attempt performing, among other things,
measurements of the local density of states (LDOS) using
the STM. The LDOS measurement using STM requires
detecting and quantifying voltage changes well below the
noise floor. The amplitude of the bias voltage changes
itself is close to the typical noise level in garden vari-
ety digital electronics. The theoretical limit to the en-
ergy resolution of the measurement, assuming an ’ideal’
lock in amplifier, is given by KbT, which at 4.2K corre-
sponds to 0.36meV and at room temperature corresponds
to 25meV. Practical measurement at the levels of the ex-
pected changes in tunneling current tend to be difficult,
since they lie below the noise floor of the system. The
’ideal’ goal of the lock-in amplifier, then, is to come as
close to this theoretical limit as possible, and if possible

6. 6
(a) Au I-V (b) Bi2Te3 I-V (c) HOPG I-V
(d) Au dI/dV vs V (e) Bi2Te3 dI/dV vs V (f) HOPG dI/dV vs V
FIG. 4. STS Results
(a) Bi2Te3 topology map
(b) Bi2Te3 conductance map
FIG. 5. Scans of Bi2Te3 in constant current mode, with cor-
responding conductance map obtained through the lock-in.
to cross it, so that the resolution is limited by the physics
of the system being measured itself. .
B. Review of lock-in techniques
The basic principle in phase sensitive detection is that
the phase and frequency information contained in the ref-
erence signal can be used to separate the signal from the
noise, which is a superimposition of sinusoids of all pos-
sible frequencies and phases at random amplitudes. The
lock-in process is easily seen mathematically as a multi-
plication of all the time domain Fourier components of
the input signal with the reference signal. The only terms
that survive subsequent time averaging are those which
contain only the components of the reference signal.
The fundamental lock-in process itself is fairly
straightforward and is a direct consequence of simple
mathematics.6
presents a frequency domain description
of the lock-in amplifier. The implementation of the lock-
in itself varies as per the design used, and the details of
process may vary slightly, but the approach in all lock in
amplifiers is fundamentally the same.
It may be noted that as long as the input signal is in
phase with the reference, the process described above
is mathematically sufficient to perform measurements.
However, the reference and signal are often out of phase
with each other. One of the major reasons for this is that
the underlying physical system takes a finite amount of
time to respond to changes in the reference frequency.
This produces a phase lag between the driving signal (the
reference) and the measurable quantity (the input to the

7. 7
lock-in).
In order to handle signals of this sort, and to accu-
rately measure the phase lag between the two signals, it
is possible to simply perform the lock-in process with the
quadrature of the reference signal. The two DC outputs
thus obtained can be used to calculate the amplitude
and phase lag of the input signal with respect to the ref-
erence. In this case also, however, the phase relationship
between the input and reference must be time invariant
for the signal to survive the final low-pass stage.
C. Lock-in amplifier design
The actual implementation of the lock-in amplifier is
generally not as straightforward as the mathematics. The
process of ’multiplying’ two signals is non-trivial. In real-
ity, the lock-in process is done using a variety of different
processes, each with its set of advantages and disadvan-
tages. Broadly, there are three different ways in which it
is implemented in literature :
1. Analog Multiplication of sines.
2. Analog Demodulation against a reference signal.
3. Manipulation of the digitized signal.
The first, the analog multiplication of sines, is the clos-
est to the mathematical formulation of the lock-in de-
scribed earlier. Multiplication of analog signals, how-
ever, is not easily achieved with discrete components.7
describes the development of a lock-in amplifier using
the AD734 multiplier from Analog Devices.
The second, analog demodulation against a reference
signal, is used more often in low-cost approaches. In
this case, the reference signal is essentially treated as a
square wave rather than as a sinusoidal wave. When the
reference signal is in its positive half cycle, the signal is
allowed through. When it is in its negative half cycle, ei-
ther the output is grounded or the signal is inverted and
allowed through. This, along with subsequent low-pass
filtering, causes the demodulation of the input signal, re-
moving the AC components added in by the reference
signal.8
describes the development of a lock in amplifier
based on Analog Devices’ AD6309
, a balanced modula-
tor/demodulator. A number of other sources have essen-
tially built on this work, including10
. This is the basis
for the design made during the course of this project.11,12
describe designs which perform essentially the same func-
tion.
The third involves digitization of the input signal, gen-
erally after some preamplification, and the digitized sig-
nal is manipulated in software. Both the above two tech-
niques are used for the calculation in software, as well
as a number of other approaches, including random sam-
pling. There has been a fair amount of contention about
whether analog or digital lock-ins are superior.13
men-
tions the disagreement. Over the years, digital lock-in
FIG. 6. Block diagram of the lock-in amplifier designed.
amplifiers have generally surpassed analog ones due to
their ability to achieve higher dynamic ranges. The in-
creasing speed and performance of digital electronics and
analog to digital converters have generally aided in their
improvement. Various approaches to digital lock-in am-
plifiers are described in literature14–16
.
A block diagram of the lock-in being developed is show
in Figure 6. The various blocks shown are functionally
distinct, and are developed individually before putting
them together as a lock in amplifier.
AC Coupling: Both the input signal as well as the ref-
erence signal are AC coupled to the circuit. This
removes any DC bias that may exist in the signal,
which would otherwise cause saturation of the in-
put amplifiers. The AC coupling is also necessary
for the lock-in ’logic’ to function as desired. In this
design, AC coupling is achieved by using a high pass
filter with a virtual ground. The design proposed
in17
may be used for greater noise immunity.
Broadband Preamplification: The second section in
the input signal’s path is the broadband pre-
amplification. INA114, a Precision Instrumenta-
tion Amplifiers is used to do this so as to protect
signal integrity while it is vulnerable to noise. In
the process, the noise contained in the input signal
is also amplified by the same amount. Multi-stage
amplification is used to overcome gain limitations
of op-amps. This allows further manipulation of
the signal easily.
Extraction of Signal from Noise: The preamplified
signal is then handled using one of the techniques
for Lock-In Amplification. The AD630 demodu-
lator is used to perform this task. The signal is
allowed to pass through only when the reference
signal is in its positive half wave. When the signal
is in its negative half wave, the output is grounded.
Components of the signal which are not related
to the reference signal die out in the time average
caused by a subsequent low pass filter, as described
previously.

8. 8
Low Pass Filter: A low pass filter is used finally to per-
form the time averaging of the AD630 output. Ad-
ditionally, a DC amplifier is used to allow ampli-
fication of the output voltage to comfortably mea-
surable levels as needed. A simple RC filter is used
to achieve this.
Reference Signal: A simple sine wave generator circuit
based on the XR2207 IC is constructed to generate
the sine wave necessary for the AC modulation. In
this design, a simple RC integrator circuit is used to
simulate the 90 degree phase shift necessary. This
approxmation is sufficient for the AD630, since all
reference signals are treated as if they are square
waves.
1. Lock-In Output
The following are the characteristics of the output of
a lock-in amplifier.
• The DC component of the input signal is discarded
completely.
• The amplitude of the signal at the reference fre-
quency, in phase with the reference shows up as
the DC output.
• The reference signal is phase shifted by 90 degrees
and the process repeated to extract the out of phase
(quardature) component.
• The amplitude and phase relationship can be ex-
tracted from the two DC outputs.
IV. CONCLUSIONS
During the course of the project, the infrastructure
necessary to perform STS using the nanoREV Air STM
was developed. Data obtained from three samples corre-
spond well to the curves expected for the three samples.
Imaging performed on the samples has been used to val-
idate the changes made to the STM during the course of
the project.
While it was initially proposed that the STS data will
be reacquired using the home-made lock-in amplifier,
that has not been possible as of yet. The lock-in am-
plifier designed has reached closer to a stage of maturity,
and testing of the device is underway. In the meanwhile,
a printed circuit board of the design has been sent for
fabrication, which should eliminate some of the uncer-
tainty regarding the sources of noise. It is expected that
the lock-in amplifier would be ready for obtaining reliable
data within the next two months.
ACKNOWLEDGMENTS
I am grateful to Prof. Deshdeep Sahdev for his guid-
ance and the opportunity to work with him on this
project. I would also like to thank Prof. Anjan Kumar
Gupta for sharing his experience with STS. I also thank
Quazar Technologies Pvt. Ltd., particularly Mr. Joshua
Mathew, for their support in making the necessary mod-
ifications to their nanoREV Air STM.
∗
chintal@iitk.ac.in; shashank.chintalagiri@gmail.com
†
ds@iitk.ac.in
1
Unknown, “Stm-sts-manipulation,” Presentation,
http://physics.units.it/Ricerca/docXXIciclo/
Microsopieavanzate/Scanning_Probe_Microscopy/
2-STS_IETS_%26_Manipulation.pdf.
2
J. Li, W.-D. Schneider, and R. Berndt, Phys. Rev. B 56,
7656 (Sep 1997).
3
H. W. Kipps, “Scanning tunneling spectroscopy : A chap-
ter in ”handbook of applied solid state spectroscopy”,”
PDF, Department of Chemistry and Materials Science Pro-
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