Thin a choice is given on the thin

Thin Film Structure Order (CdS/4-CTP/Au/substrate):
Tasked with the
production of a thin film structure consisting of a 5 nm CdS layer, a
4-carboxythiophenol (4-CTP) layer, and a 5 nm gold (Au) layer on an insulating
substrate, a choice is given on the thin film structure of either
Au/4-CTP/CdS/substrate or CdS/4-CTP/Au/substrate. In literature, it has been
shown that the orientation of certain self-assembled monolayers (SAMs) such as
4-CTP can be dependent on the surface used1. For a bifunctional SAM such as 4-CTP, which has both
a thiol and a carboxyl functional group, choosing a Au surface will often result
in carboxy terminated SAMs where a thiol-gold bond is formed. Also, given the need
for the surface layer to be of high quality in order to ensure uniform SAM
coverage, it would be prudent to use Au as the surface instead of CdS. Since
CdS is a compound semiconductor, it can, depending on the synthesis method,
suffer from defects and contamination which can result in non-uniform formation
of a 4-CTP layer. Using a CdS/4-CTP/Au/substrate scheme would provide the most optimal
thin film structure.

Au Synthesis: Two of the methods that can be used
to synthesize an Au thin film on an insulating substrate are electron-beam
evaporation (EBE) and chemical vapor deposition (CVD). EBE is a form of physical
vapor deposition (PVD) which uses an electron beam to heat the target material
(Au), typically under high-vacuum, to the point in which it transforms into its
gaseous phase. These atoms would then travel upwards and precipitate on the substrate
resulting in the formation of a thin film2. One of the main advantages of EBE is low
contamination which would allow for a high purity thin film. But it also has
the disadvantage of generating electron radiation, which can often irradiate
the substrate/sample upon which the target material is deposited2. For CVD, in contrast to EBE, synthesis of a thin
film is achieved through chemical reaction, not physical deposition. In a CVD
chamber, the substrate is exposed to a precursor(s) which then breaks down or
reacts with the substrate surface to form the desired thin film. Any volatile
precursor reaction products are evacuated out of the chamber2. A major advantage of CVD is precise control of film
thickness, which can be extremely useful in achieving a 5 nm continuous Au thin
film3. One of the main disadvantages is that in a CVD
process the precursors can be toxic and quite costly, and thin film deposition
usually done at high temperature4.

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Au Analysis: Atomic force microscopy (AFM) and grazing
incidence x-ray diffraction (GIXRD) are powerful techniques that can be used to
analyze the Au thin film. AFM is a scanning probe microscopy (SPM) technique that
uses a cantilever which oscillates at a certain frequency with a laser beam
pointed at the tip of the cantilever in order to measure its deflection. When contact
is made with the sample surface, the interactions between the tip of the
cantilever and the surface is recorded by changes in cantilever height or
frequency5. These recorded changes can be used to from a
topographical image. Overall, AFM can be used to raster the tip across a thin
film surface in order to form an image that provide information on thin film morphology
and coverage. A big advantage of AFM is that it does not require a vacuum or any
sort of treatment that may damage the sample. Some drawbacks of AFM include a
relatively slow scan time as well as a small scan image size. X-ray diffraction
(XRD) is a technique which uses a beam of monochromatic x-rays that are incident
on the sample surface where these x-rays undergo scattering which can be used
to determine crystallinity, grain size, and other physical properties of thin
films6. Using a grazing angle allows the study of thin films
where surface sensitivity is important. Some advantages of GIXRD is that it is
a non-destructive technique without a need for a transparent substrate and mostly
avoids any scattering from the substrate7. Some disadvantages are that x-rays do not interact
strongly with lighter elements and that there are some size limitations where
it is more accurate in measuring large crystalline structures than small ones
which can often go undetected in trace amounts.

4-CTP Synthesis: Two of the methods that can be used
to synthesize the 4-CTP layer are the Langmuir-Schaefer (LS) method and the
Langmuir-Blodgett (LB) method. The LS deposition method involves the dipping of
the substrate into a solution that has the 4-CTP monolayer and simply making
contact with the layer parallel to the substrate. The LB deposition method is
similar to the LS method in that it involves the dipping of a substrate into a
solution that has a monolayer in it. The LB method uses a trough to maintain
constant surface pressure while dipping the substrate at a right angle to the
SAM8. Both techniques have the advantage of being liquid
based deposition methods that do not require complicated setups nor any sort of
vacuum system, and they can both be used to form multilayers. The major
disadvantage of the LS method is that the correct head group, in this case the
thiol group, must be facing upwards in order to make the desired bond with the Au
surface. The major disadvantage of the LB method is that both sides of the
substrate are coated with the layer which may not always be desirable.

4-CTP Analysis: Angle-resolved x-ray photoelectron
spectroscopy (ARXPS) and a scanning electron microscope (SEM) can be used to
analyze the 4-CTP layer. The ARXPS can be utilized as a non-destructive depth
profiling technique that uses XPS at different take-off angles in order to
qualitatively obtain the layer thickness and orientation9. XPS itself is a characterization technique that uses
energetic x-rays to impact the sample surface causing the emission of
photoelectrons. Then an energy analyzer is used to measure their kinetic energy
and from the binding energy spectra the elemental identification and
quantification can be obtained6. In the case on non-destructive ARXPS, the advantage
of non-destructive depth profiling is offset by the difficulty of obtaining
quantitative data of the sample9. The SEM is a powerful technique that produces images
of a sample by scanning the sample surface with a beam of electrons6. It can be used to obtain high resolution,
three-dimensional topological images. It can also be used to ascertain the
coverage of the 4-CTP layer on Au. SEM is a relatively fast technique with
minimal sample preparation required. Some of the disadvantages of an SEM are
its high cost and large, complex setup.

CdS Synthesis: Two of the methods that can be used
to synthesize a CdS film are molecular beam epitaxy (MBE) and pulsed laser
deposition (PLD). MBE is a technique that uses epitaxial growth to grow a
single crystal system one layer at a time. This is achieved through the use
effusion cells which shoot a different type of species at the substrate where
they land and condense to form the thin layers6. Major advantages of MBE are the precise thickness
control, extremely low contamination, and highly controlled growth conditions.
Some drawbacks are its extremely slow growth rate and very expensive equipment2. PLD, which involves a very complex process, can be
summed up as a deposition method that uses a high-power laser to strike a
target in order to vaporize the desired material. Then this plasma plume
deposits unto a substrate in order to attain a thin film2. The major advantage of PLD over other PVD techniques
is that the produced plume of material has the same stoichiometry as the
target, allowing the user to obtain the desired stoichiometry for a compound
like CdS. PLD is relatively fast and versatile where various materials can be
deposited, but the kinetic energies of some species can cause defects in the
growing film.

CdS Analysis:  Reflection
high-energy electron diffraction (RHEED) and secondary ion mass spectrometry
(SIMS) can be used to characterize the CdS thin film. RHEED is commonly used in
MBE systems to monitor the growth of the structures2. It is a surface sensitive technique that uses an
electron beam at a grazing angle to measure the surface periodicity through the
interpretation of a steak pattern7. A major advantage of RHEED is the monitoring of atomic
layer by layer growth, which is highly desirable for growth of a high-quality,
5 nm CdS thin film. The surface sensitivity of RHEED can be a drawback as it
requires low roughness of thin film surfaces. SIMS is an extremely powerful technique
that uses high energy ions to impact the sample surface and eject secondary
ions for analysis. A mass analyzer is used to measure the mass/charge ratios of
the secondary ions in order to determine the elemental composition of the
surface10. SIMS is a highly sensitive technique
that can be used to detect any sort of CdS thin film contamination and even
perform depth profiling by recording sequential SIMS spectra. The major
disadvantage of SIMS is that it is a highly destructive technique.

References:

(1)       Jadhav, S. A. Cent.
Eur. J. Chem. 2011, 9 (3), 369–378. (2)       MSEN 5361. Fall 2017, Lecture 3.

(3)       Parkhomenko,
R. G.; Morozova, N. B.; Zharkova, G. I.; Shubin, Y. V.; Trubin, S. V.;
Kriventsov, V. V.; Kuchumov, B. M.; Koretskaya, T. P.; Igumenov, I. K. Chem.
Vap. Depos. 2012, 18 (10–12), 336–342.

(4)       Creighton, J.
R.; Ho, P. Chem. Vap. Depos. 2001, 1–13. (5)     MSEN 5361. Fall 2017, Lecture 10.

(6)       Vickerman,
John C.; Gilmore, I. S. Surface Analysis: The Principal Techniques;
Wiley, 2009.

(7)       MSEN 5361.
Fall 2017, Lecture 9.          (8)        MSEN 5361. Fall 2017, Lecture 4.

(9)       MSEN 5361.
Fall 2017, Lecture 5.          (10)      MSEN 5361. Fall 2017, Lecture 11.

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