Semiconductor , underpotential

Generally, the experimental results on electrodeposition of CdS in acidic solutions of thiosulfate have implied that CdS growth does not involve underpotential deposition of the less noble element (Cd), as would be required by the theoretical treatments of compound semiconductor electrodeposition. Hence, a fundamental difference exists between CdS and the other two cadmium chalcogenides, CdSe and CdTe, for which the UPD model has been fairly successful. Besides, in the present case, colloidal sulfur is generated in the bulk of solution, giving rise to homogeneous precipitation of CdS in the vessel, so that it is quite difficult to obtain a film with an ordered structure. The same is true for the common chemical bath CdS deposition methods. 

Chenthamarakshan CR, Ming Y, Rajeshwar K (2000) Underpotential photocatalytic deposition A new preparative route to composite semiconductors. Chem Mater 12 3538-3540... 

UPD Cd can also be used to obtain cadmium sulfide, an important semiconductor for electronics. Electrochemical epitaxial growth of organized CdS structures, involving underpotentially deposited Cd on Au(lll) was thus reported [161, 265]. 

Scanning tunneling microscopy (STM), 787. 1157 bioelectrochemistry and, 1159 electrochemistry and. 1158 electrodeposition and. 1310 nanotechnology, 1345 piezoelectric crystal, 1158 tunneling current. 1157 underpotential deposition, 1313, 1315 Scavanger electrolysis, electrodeposition, 1343 Schlieren method, diffusion layer. 1235 Schmickler, 1495,1510 Schrodinger equation, 1456, 1490 Schultze 923,1497.1510 Screw dislocation, 1303, 1316, 1321, 1326 Secondary reference electrode, 815, 1109 Self-consumed electrode, 1040 Semiconductors... 

CdTe, a II—VI compound semiconductor with a direct band gap of 1.44 eV at room temperature, is, from its physical properties, a promising photovoltaic material. The electrodeposition of CdTe in ionic liquid was published recently by Sun et al. [38]. They were able to show that the semiconductor can be electrodeposited at elevated temperature (above 120 °C) in the Lewis basic l-ethyl-3-methylimidazolium chloride/tetrafluoroborate ionic liquid containing CdCh and TeCU. CdTe films were obtained by the underpotential deposition (UPD) of Cd on the deposited Te. The deposit composition was independent of the deposition potential within the Cd UPD regime. The crystallinity of the deposits is improved by increasing the deposition temperature, which again demonstrates the high potential of the wide thermal windows of ionic liquids for compound electrodeposition. 

Atomic layer epitaxy (electrochemical) — Electrochemical atomic layer epitaxy (ECALE) is a self-limiting process for the formation of structurally well-ordered thin film materials. It was introduced by Stickney and coworkers [i] for the layer by layer growth of compound semiconductors (CdTe, etc.). Thin layers of compound semiconductors can be formed by alternating - underpotential deposition steps of the individual elements. The total number of steps determines the final thickness of the layer. Compared to flux-limited techniques... 

Layered nanostructures can be deposited from the electrochemical environment by applying a time dependent voltage program to the working electrode (5) or by using a sequential deposition scheme such as electrochemical atomic layer epitaxy (EC-ALE) (6-10). In EC-ALE, a surface-limited electrochemical reaction, such as underpotential deposition (upd), is used to synthesize a binary compound by successive deposition of each element from its respective solution precursor. EC-ALE is an attractive electrosynthetic alternative to conventional deposition methods that is inexpensive, operates at ambient temperature and pressure and provides precise film thickness control. This technique promises to overcome many problems associated with other electrosynthetic approaches, such as the formation of highly polycrystalline deposits and interfacial interdiffusion. For example, we have recently used EC-ALE to fabricate stable semiconductor heterojunctions with extremely abrupt interfaces (11). 

The second example involves the surface chemistry of the compound semiconductor CdSe synthesized epitaxially on Au(100) by underpotential deposition (UPD). By analogy with the gas-phase epitaxial deposition procedure, this UPD-based method has been dubbed electrochemical atomic layer epitaxy (ECALE) [6]. Unique information on the interfacial structure of the first adlayer of Se electrodeposited was revealed by STM experiments. 

The study of metal adatoms deposited onto the surface of semiconductor electrodes is of interest for electrodeposition of epitaxial [1] and textured [2] films and superlattices of metal chalcogenides [3]. In the past decade, an additional interest in processes of underpotential deposition (upd) has been stimulated by the progress in studying physics and chemistry of nanosize objects [4]. 

Specific adsorption at electrochemical interfaces. Bragg diffraction XSW has been used to probe the structures of specifically adsorbed ions at metal-electrolyte and semiconductor-electrolyte interfaces under controlled electrochemical conditions. Among the earliest work in this category is the study of T1 adsorption from a dilute solution (mM) on Cu(l 11) through underpotential deposition (Materlik et al. 1987 Zegenhagen et al. 1990). 

The direct electrodeposition of GaAs from ionic liquids has been studied mainly by two groups. Wicelinski et al. [66] used an acidic chloroaluminate melt at 35-40 °C to co-deposit Ga and As. However, it was reported that Al underpotential deposition occurs on Ga. Carpenter et al. employed an ionic liquid that was based on GaQs to which AsQs was added [18,67]. Unfortunately, in these studies the quality of the deposits was not convincing and both pure arsenic and gallium could be found in the deposits. Nevertheless, these studies have to be regarded as the first steps in the electrodeposition of Ga-based semiconductors. Furthermore, thermal annealing could improve the quality of the deposits. 

Second harmonic generation has also been used to study the semiconductor/ solution interface during the deposition of gold on Si(lll) [777]. In a setup combining SHG and the electrochemical quartz crystal microbalance (EQCM), the underpotential deposition of copper on a polycrystalline gold surface has been studied a decrease of the SHG signal by 60% upon formation of the upd-layer was found [778]. A study of the electrochemical liquid/liquid interface between two immiscible solutions where adsorption of surfactants occurred has been reported [779]. 

This chapter concerns the state of development of electrochemical atomic layer epitaxy (EC-ALE), the electrochemical analog of atomic layer epitaxy (ALE). EC-ALE is being developed as a methodology for the electrodeposition of compound semiconductors with nanoscale control. ALE is based on the formation of compounds, one monolayer (ML) at a time, using surface-limited reactions. An atomic layer of one element can be electrodeposited at a potential under that needed to deposit the element on itself, and this process is referred to as underpotential deposition (UPD). EC-ALE is the use of UPD for the surface-limited reactions in an ALE cycle. 

Fundamental Thermodynamic Aspects of the Underpotential Deposition of Hydrogen, Semiconductors, and Metals... 

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