Current Research

A schematic representation of atomic layer etching mechanisms of iron surface. The reactions of acetylacetone with clean iron surface (a), chlorine with the same surface (b) or chlorine with oxidized iron (c) do not result in iron removal at processing temperature. Reactions of acacH with chlorinated iron (d) or with a surface that is chlorinated and oxidized (e) can be optimized to result in iron removal one atomic layer at a time.

Atomically Precise Deposition and Etching

The technological process involving atomically-precise layer-by-layer removal of materials is referred to as atomic layer etching (ALE). This approach is often used to achieve uniform ultra-thin films with atomically precise thickness, especially for materials that are difficult to form by the more traditional techniques that rely on various deposition processes. The ALE process is often considered to be a reverse of the more common atomic layer deposition (ALD), with both approaches rooted in sequential self-limiting surface reactions. In ALD, each steps of the process saturates the monolayer with the species of a specific chemical structure and reactivity that selectively reacts with the species of the second half of the cycle to deposit target material. In ALE, the goal is to saturate the monolayer with the species of one type and then react them with the reagent used in the second half-cycle to remove a monolayer of material. Of course, each cycle of both ALE and ALD can in general involve more than two steps. If only temperature and pressure (concentration) are used as variables for the etching process, it is often referred to as thermal dry etching. The disadvantage of this approach is that most of the time the process is not driven thermodynamically and the thermodynamic requirements have to be overcome kinetically. In other words, one of the half-cycles has to be stopped at a precise surface concentration of specific species rather than rely fully on the thermodynamically driven single molecular layer saturation at reasonable processing conditions. However, the advantage of using ALE, specifically for magnetic materials, is in eliminating slow nucleation step (that often leads to surface roughening) and moreover, in smoothing the surface of the etched film by ALE that has been reported. Thermal ALE process also eliminates the need for expensive stimulation approaches often needed for deposition, such as plasma, that are also often responsible for surface damage. We are studying the molecular mechanisms of both processes and collaborate with other research groups and industrial partners on applications of our findings.

Relevant Publications:

  1. Konh, M., Lien, C., Cai, X., Wei, S.-H., Janotti, A., Zaera, F, and Teplyakov, A. V. ToF-SIMS Investigation of the Initial Stages of MeCpPt(CH3)3 Adsorption and Decomposition on Nickel Oxide Surfaces: Exploring the Role and Location of the Ligands. Organometallics 2020, 39(7), 1024-1034, DOI:10.1021/acs.organomet.9b00781.
  2. Konh, M., Lien, C., Zaera, F, and Teplyakov, A. V. Application of Time-of-Flight Secondary Ion Mass Spectrometry to the Detection of Surface Intermediates during the First Cycle of Atomic Layer Deposition (ALD) of Platinum on Silica Surfaces. Appl. Surf. Sci. 2019, 488, 468-476, DOI: 10.1016/j.apsusc.2019.05.209.
  3. Konh, M., He, C., Lin, X., Guo, X., Pallem, V., Opila, R. L., Teplyakov, A. V., Wang, Z., and Yuan, B. Molecular Mechanisms of Atomic Layer Etching of Cobalt with Molecular Chlorine and Diketones. J. Vac. Sci. Technol. A. 2019, 37, 021004; DOI: 10.1116/1.5082187. This article was chosen it to be promoted as an Editor’s Pick.
  4. Zhao, J., Konh, M., and Teplyakov, A. V. Surface Chemistry of Thermal Dry Etching of Cobalt Thin Films Using Hexafluoroacetylacetone (hfacH). Appl. Surf. Sci. 2018, 455, 438-445, DOI: 10.1016/j.apsusc.2018.05.182.

 

Surface Modification of Semiconductor Materials

Summary of the approaches to form a Si−N bond on silicon surfaces with N-containing compounds.

Semiconductor substrates are widely used in many applications. Multiple practical uses involving these materials require the ability to tune their physical (bandgap, electron mobility) and chemical (functionalization, passivation) properties to adjust those to a specific application. The goal of this research direction is to develop new strategies for manipulating the surface properties of semiconductor materials in a controlled way. Our expertise allows us to selectively tune the chemical and physical properties of semiconductor surfaces (both flat or nanostructured, where preservation of surface morphology is a complex issue) by an appropriate choice of elemental, II-VI, III-V semiconductor, or by chemical modification. Our approach focuses on chemical passivation, on molecular switches and on the use of a variety of functionalized self-assembled monolayers. The findings of these investigations will be relevant for future applications in molecular and nanoelectronics, sensing, and solar energy conversion.

Relevant Publications:

  1. He, C., Cai, X., Wei, S.-H., Janotti, A., and Teplyakov, A. V. Self-Catalyzed Sensitization of CuO Nanowires via Solvent-Free Click Reaction. Langmuir 2020, DOI: 10.1021/acs.langmuir.0c02262.
  2. He, C., Janzen, R., and Teplyakov, A. V. ‘Clickable’ Metal Oxide Nanomaterials Surface-Engineered by Gas-Phase Covalent Functionalization with Prop-2-ynoic Acid. Chem. Mater. 2019, 31, 2068-2077, DOI: 10.1021/acs.chemmater.8b05124.
  3. He, C. and Teplyakov, A. V. 29,31-H Phthalocyanine Covalently Bonded Directly to a Si(111) Surface Retains Its Metalation Ability. Langmuir 2018, 34 (37), 10880–10888, DOI: 10.1021/acs.langmuir.8b02259.
  4. He, C., Abraham, B., Fan, H., Harmer, R., Li, Z., Galoppini, E., Gundlach, L, and Teplyakov, A. V. Morphology-Preserving Sensitization of ZnO Nanorod Surfaces via Click-Chemistry. J. Phys. Chem. Lett. 2018, 9(4), 768-772, DOI: 10.1021/acs.jpclett.7b03388.

 

Monolayer Modification for Ultrashallow Doping of Semiconductors

Schematic representation of the monolayer doping approach to introduce nitrogen to silicon surface via a reaction of functionalized silicon with hydrazine

Selectively terminated (or functionalized) silicon surfaces serve as a golden standard for chemical modification of semiconductors from both fundamental and applied points of view. Numerous reviews have highlighted the key points in the way these single crystalline surfaces are modified and used in practical applications. Doping silicon surfaces, especially ultra-shallow doping with high dopant concentrations, is at the core of many of these applications. Monolayer doping has been used to achieve shallow doping (< 5nm) of silicon with relatively high dopant concentrations (> 1020 cm-3) through a process by which the silicon surface is functionalized with a dopant containing molecule followed by a rapid thermal anneal (RTA) to drive the dopant into the substrate. Monolayer doping provides a relatively quick and low-cost wet chemistry-based method to dope silicon substrates without having to work in high vacuum environments and to control the dopant concentration by chemical design. We are working on understanding molecular mechanisms of monolayer doping processes and surface functionalization approaches. we collaborate with other groups to understand the difference in chemistry of ideal surfaces prepared in ultra-high vacuum and the ones prepared by solution chemistry methods.

Relevant Publications:

  1. Silva-Quinones, D., He, C., Dwyer, K. J., Butera, R. E., Wang, G. T., and Teplyakov, A. V. Reaction of Hydrazine with Solution- and Vacuum-Prepared Selectively Terminated Si(100) Surfaces: Pathways to the Formation of Direct Si-N Bonds. Langmuir 2020, 36, 12866-12876,  DOI: 10.1021/acs.langmuir.0c02088.
  2. Silva-Quinones, D., He, C., Butera, R. E., Wang, G. T., and Teplyakov, A. V. Reaction of BCl3 with H- and Cl-Terminated Si(100) as a Pathway for Selective Monolayer Doping through Wet Chemistry. Appl. Surf. Sci. 2020, 533, 146907, DOI: 10.1016/j.apsusc.2020.146907.

Monolayer Aspects of Nanocatalysis for Dry Methane Reforming and other Hydrocarbon Processing

Platinum-based supported catalysts for hydrocarbon conversion are among the most effective for selective dehydrogenation and isomerization processes, However, high process temperatures and possibility of coke formation require catalyst modifications to mitigate such effects. One of the emerging approaches to prevent platinum catalyst deactivation is the use of boron additives that have been proposed to prevent coking. Another approach is to replace platinum with nanoparticulate catalysts based on cheaper transition methods. We use multiple spectroscopic and microscopic techniques to understand the properties of these materials and the roles played by their components in transformation of hydrocarbons.

Relevant Publications:

  1. Rani, S., Byron, C., and Teplyakov, A. V. Formation of Silica-Supported Platinum Nanoparticles as a Function of Preparation Conditions and Boron Impregnation. J. Chem. Phys. 2020, 152, 134701, DOI:10.1063/1.5142503.
  2. Byron, C., Bai, S., Celik, G., Ferrandon, M. S., Liu, C., Ni, C., Mehdad, A., Delferro, M., Lobo, R. F., and Teplyakov, A. V. Role of Boron in Enhancing the Catalytic Performance of Supported Platinum Catalysts for Non-Oxidative Dehydrogenation of n-Butane. ACS Catal. 2020, 10, 1500-1510, DOI: 10.1021/acscatal.9b04689.

We also work with chemometrics, data evaluation:

Multivariate Curve Resolution analysis of thermal desorption data (click to see full manual)

The substantial amount of information carried in temperature-programmed desorption (TPD) experiments is often difficult to mine due to the occurrence of competing reaction pathways that produce compounds with similar mass spectrometric features. Multivariate curve resolution (MCR) is introduced as a tool capable of overcoming this problem by mathematically detecting spectral variations and correlations between several m/z traces, which is later translated into the extraction of the cracking pattern and the desorption profile for each desorbate. Different from the elegant (though complex) methods currently available to analyze TPD data, MCR analysis is applicable even when no information regarding the specific surface reaction/desorption process or the nature of the desorbing species is available. See the SAMPLE TPD ANALYSIS FILE and the MCR MANUAL available online.

Relevant Publication:

  1. Rodríguez-Reyes, J. C. F., Teplyakov, A. V. and Brown, S. D. Qualitative and quantitative analysis of complex temperature-programmed desorption data by multivariate curve resolution. Surf. Sci. 2010.