Thin a choice is given on the thin

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Last updated: December 24, 2019

Thin Film Structure Order (CdS/4-CTP/Au/substrate):Tasked with theproduction of a thin film structure consisting of a 5 nm CdS layer, a4-carboxythiophenol (4-CTP) layer, and a 5 nm gold (Au) layer on an insulatingsubstrate, a choice is given on the thin film structure of eitherAu/4-CTP/CdS/substrate or CdS/4-CTP/Au/substrate. In literature, it has beenshown that the orientation of certain self-assembled monolayers (SAMs) such as4-CTP can be dependent on the surface used1. For a bifunctional SAM such as 4-CTP, which has botha thiol and a carboxyl functional group, choosing a Au surface will often resultin carboxy terminated SAMs where a thiol-gold bond is formed. Also, given the needfor the surface layer to be of high quality in order to ensure uniform SAMcoverage, it would be prudent to use Au as the surface instead of CdS. SinceCdS is a compound semiconductor, it can, depending on the synthesis method,suffer from defects and contamination which can result in non-uniform formationof a 4-CTP layer. Using a CdS/4-CTP/Au/substrate scheme would provide the most optimalthin film structure. Au Synthesis: Two of the methods that can be usedto synthesize an Au thin film on an insulating substrate are electron-beamevaporation (EBE) and chemical vapor deposition (CVD). EBE is a form of physicalvapor 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 itsgaseous phase.

These atoms would then travel upwards and precipitate on the substrateresulting in the formation of a thin film2. One of the main advantages of EBE is lowcontamination which would allow for a high purity thin film. But it also hasthe disadvantage of generating electron radiation, which can often irradiatethe substrate/sample upon which the target material is deposited2. For CVD, in contrast to EBE, synthesis of a thinfilm is achieved through chemical reaction, not physical deposition.

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In a CVDchamber, the substrate is exposed to a precursor(s) which then breaks down orreacts with the substrate surface to form the desired thin film. Any volatileprecursor reaction products are evacuated out of the chamber2. A major advantage of CVD is precise control of filmthickness, which can be extremely useful in achieving a 5 nm continuous Au thinfilm3. One of the main disadvantages is that in a CVDprocess the precursors can be toxic and quite costly, and thin film depositionusually done at high temperature4.Au Analysis: Atomic force microscopy (AFM) and grazingincidence x-ray diffraction (GIXRD) are powerful techniques that can be used toanalyze the Au thin film. AFM is a scanning probe microscopy (SPM) technique thatuses a cantilever which oscillates at a certain frequency with a laser beampointed at the tip of the cantilever in order to measure its deflection. When contactis made with the sample surface, the interactions between the tip of thecantilever and the surface is recorded by changes in cantilever height orfrequency5.

These recorded changes can be used to from atopographical image. Overall, AFM can be used to raster the tip across a thinfilm surface in order to form an image that provide information on thin film morphologyand coverage. A big advantage of AFM is that it does not require a vacuum or anysort of treatment that may damage the sample. Some drawbacks of AFM include arelatively 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 incidenton the sample surface where these x-rays undergo scattering which can be usedto determine crystallinity, grain size, and other physical properties of thinfilms6. Using a grazing angle allows the study of thin filmswhere surface sensitivity is important. Some advantages of GIXRD is that it isa non-destructive technique without a need for a transparent substrate and mostlyavoids any scattering from the substrate7. Some disadvantages are that x-rays do not interactstrongly with lighter elements and that there are some size limitations whereit is more accurate in measuring large crystalline structures than small oneswhich can often go undetected in trace amounts. 4-CTP Synthesis: Two of the methods that can be usedto synthesize the 4-CTP layer are the Langmuir-Schaefer (LS) method and theLangmuir-Blodgett (LB) method. The LS deposition method involves the dipping ofthe substrate into a solution that has the 4-CTP monolayer and simply makingcontact with the layer parallel to the substrate. The LB deposition method issimilar to the LS method in that it involves the dipping of a substrate into asolution that has a monolayer in it. The LB method uses a trough to maintainconstant surface pressure while dipping the substrate at a right angle to theSAM8.

Both techniques have the advantage of being liquidbased deposition methods that do not require complicated setups nor any sort ofvacuum system, and they can both be used to form multilayers. The majordisadvantage of the LS method is that the correct head group, in this case thethiol group, must be facing upwards in order to make the desired bond with the Ausurface. The major disadvantage of the LB method is that both sides of thesubstrate are coated with the layer which may not always be desirable. 4-CTP Analysis: Angle-resolved x-ray photoelectronspectroscopy (ARXPS) and a scanning electron microscope (SEM) can be used toanalyze the 4-CTP layer. The ARXPS can be utilized as a non-destructive depthprofiling technique that uses XPS at different take-off angles in order toqualitatively obtain the layer thickness and orientation9. XPS itself is a characterization technique that usesenergetic x-rays to impact the sample surface causing the emission ofphotoelectrons. Then an energy analyzer is used to measure their kinetic energyand from the binding energy spectra the elemental identification andquantification can be obtained6.

In the case on non-destructive ARXPS, the advantageof non-destructive depth profiling is offset by the difficulty of obtainingquantitative data of the sample9. The SEM is a powerful technique that produces imagesof 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 thecoverage of the 4-CTP layer on Au. SEM is a relatively fast technique withminimal sample preparation required. Some of the disadvantages of an SEM areits high cost and large, complex setup.

CdS Synthesis: Two of the methods that can be usedto synthesize a CdS film are molecular beam epitaxy (MBE) and pulsed laserdeposition (PLD). MBE is a technique that uses epitaxial growth to grow asingle crystal system one layer at a time. This is achieved through the useeffusion cells which shoot a different type of species at the substrate wherethey land and condense to form the thin layers6. Major advantages of MBE are the precise thicknesscontrol, 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 besummed up as a deposition method that uses a high-power laser to strike atarget in order to vaporize the desired material. Then this plasma plumedeposits unto a substrate in order to attain a thin film2. The major advantage of PLD over other PVD techniquesis that the produced plume of material has the same stoichiometry as thetarget, allowing the user to obtain the desired stoichiometry for a compoundlike CdS.

PLD is relatively fast and versatile where various materials can bedeposited, but the kinetic energies of some species can cause defects in thegrowing film.CdS Analysis:  Reflectionhigh-energy electron diffraction (RHEED) and secondary ion mass spectrometry(SIMS) can be used to characterize the CdS thin film. RHEED is commonly used inMBE systems to monitor the growth of the structures2. It is a surface sensitive technique that uses anelectron beam at a grazing angle to measure the surface periodicity through theinterpretation of a steak pattern7. A major advantage of RHEED is the monitoring of atomiclayer 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 itrequires low roughness of thin film surfaces. SIMS is an extremely powerful techniquethat uses high energy ions to impact the sample surface and eject secondaryions for analysis. A mass analyzer is used to measure the mass/charge ratios ofthe secondary ions in order to determine the elemental composition of thesurface10.

SIMS is a highly sensitive techniquethat can be used to detect any sort of CdS thin film contamination and evenperform depth profiling by recording sequential SIMS spectra. The majordisadvantage 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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