Strap in ― we are about to go for a ride.
Our world stands at the threshold of the next technological revolution. And like electronics that drove the current personal computers and cell phones technologies, it's guaranteed to drastically change the way we live.
Electronics helped fuel the revolution of automation and information technology. Computers once occupied rooms filled with vacuum tubes and circuitry. Then integrated circuits with thousands, then millions of transistors allowed engineers to shrink the size of computers in the 1960s. Those morphed into desktop tools and pocket devices during the following decades.
This miniaturization followed Moore's Law, which says the number of transistors packed on an integrated circuit doubles every 18 months ― reducing the cost proportionately.
Moore's Law lasted about 50 years. We have now reached the limit of how small we can shrink these electronics, fabricated from bulk silicon semiconductors. That ceiling is called the quantum limit, the smallest possible size that still poses bulk properties and beyond which fascinating quantum phenomena emerge.
Just ask Masoud Mahjouri-Samani, Ph.D. an assistant professor of electrical and computer engineering at Auburn University who studies laser-based synthesis, laser processing, and in-situ laser diagnostics of emerging materials and devices.
“So we ask, OK, what can we do if the current technology cannot be scaled any further?” Mahjouri-Samani said. “The answer is, we should look for new materials that have new properties beyond what we've been expecting from the traditional bulk materials.”
These new low-dimensional substances are called quantum materials, where the effects of reduced screening and quantum confinement give rise to exotic and often unexpected properties, with no counterpart in the macroscopic world. Examples of such low-dimensional materials are atomically thin two-dimensional (2D) layered crystals, which exhibit properties not present on the traditional bulk 3D crystals.
“Two-dimensional materials are atomically thin sheets with only a single or few atoms in the “Z” dimension, meaning charges (wave functions) can only move in X and Y dimension, depending on the crystal symmetry and are confined in the Z direction.” Mahjouri-Samani said. “The surface atoms are naturally terminated and chemically inert, which define their electronic structure and constrain their interaction with the surrounding medium or materials. Such characteristics make them essentially different from just thinning down a bulk 3D crystal.”
“In bulk 3D crystals, there are all these dangling bonds on the surface that want to bind to more atoms and grow larger,” he said. “So if you shrink that 3D material to a smaller and a smaller dimension, you will end up having a structure that is all surface with tons of dangling bonds that generate a lot of unwanted electronic states within your material. Surface passivation could help fixing these dangling bonds, but it might introduce even more complexity into the system. This makes the emerging 2D layered materials an interesting platform to work with.”
“A large selection of 2D materials are already available in nature,” he said. “For example, graphene is a single sheet of graphite, and by nature, a single atomic layer. But even graphene has issues when it comes to electronics, optoelectronics and photonics devices. It behaves like a semimetal. So a new class of 2D materials was discovered, called transition metal dichalcogenides (TMDC), where a single atomic sheet of transition metal is sandwiched between two atomic sheets of chalcogen atoms.”
TMDCs exhibit a unique combination of atomic-scale thickness, direct bandgap, strong spin-orbit coupling and favorable electronic and mechanical properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine[i].
“Researchers can manipulate atomic structure of these crustal, for example, by placing a dopant atom or a defect where desired to intentionally create quantum states of interest and manipulate the way electrons and spin behave in the system” he said. “We can do a lot of interesting things with these materials.”
The next problem is how to make these structures. Scientists need to be able to control the crystalline quality, amount of defects, vacancies, and dopants in these crystals. Some are using chemical vapor deposition and other techniques.
Mahjouri-Samani has found an alternative way to advance 2D quantum materials research and discovery. He uses spatial and temporal tunability, controlled energy and power densities, adjustable bandwidth, and polarization of lasers to synthesize, process, and characterize the 2D materials. [ii][iii][iv][v]For example, he uses laser-based synthesis for time-resolved growth of high-quality crystals, uses laser processing to induce defects or perform localized doping in 2D materials, and uses laser diagnostic for in-situ and ex-situ monitoring of their properties.[vi]
He needs to be able to monitor the structural and electronics changes of crystals when he dopes or creates defects in these crystals. For example, monitoring the structural changes with the Raman and electronics changes with the photoluminescence spectroscopy.
In these new materials, “the crystals vibrations or so-called Raman fingerprints are very well-defined,” he said. “So with any changes in the quality, type, and arrangements of the atoms in the system, the Raman vibration frequencies start shifting so you can monitor it. Similarly, PL emission shifts if the crystal is manipulated. That's why laser-based diagnostics is key to monitor what is going on in these materials right now.”
As is typical for a scientist, Mahjouri-Samani customized an instrument to meet his particular needs. He integrated Horiba iHR320 spectrometer, EMCCD, and PMT with a customized microscope to perform Raman, photoluminescence (PL) and TCSPC, or time-correlated single-photon counting.
He added a couple of columns on top of the instrument, the optical path and the laser processing and diagnostic path which are aligned to go through the microscope to the sample and then back to the spectrometer. He calls it a laser diagnostic system.
“We can monitor those vibrations and PL emissions,” he said. “It will help us understand what happens to their structure when manipulated”.
Controlling the sample is important, and you need to know whether the sample is doped or has defects as intended to create a material that is useful for future applications.
“That’s why Raman and PL spectroscopy is a perfect tool,” he said.
Transmission electron microscopes can also be used to perform the analysis, but these are costly, time-consuming and require sample preparation. Raman and PL spectroscopies are quicker, and allows the researcher to do their analysis in real-time without any sample preparation and contamination issues.
“We are able to synthesize various types of 2D materials,” he said. “Now we are able to process them with lasers. We dope them, we create defects, and then use the Raman and PL and lifetime to study what happens, as we control the process.”
The goal of Masoud Mahjouri-Samani's research is to make better quantum and functional materials for future applications. These materials may bridge the gap between our current electronics and a future of unimaginable technologies. But it will take the discovery and development of these new materials to make that leap.
“We are right at the edge of the technological revolution to a new way of making materials and devices, and now we are going toward quantum information sciences, quantum electronics, quantum photonics, and so on because we are reaching to that regime of dimensionality,” Mahjouri-Samani prophesized.
The focus nowadays is on the fundamental research to understand how these materials are behaving, to understand how we can make and manipulate these materials to control their behavior.
“Just like silicon 50 years ago, all the effort was on how we can make single crystals, how we can make the crystals as large as possible, how we can dope them with donors and acceptors, and how we can create hybrid and heterostructures between them to create these technologies we have today,” he said.
“Now we are focusing on how we can make single crystals of these quantum materials, how we can manipulate them, how we can tailor their properties, how we can create heterostructures among them, how can we make them compatible with the current manufacturing infrastructure, and how we can combine them with the other materials to make 0D, 1D, 2D, and 3D Hybrid structures.”
It’s all in the fundamental research and development stage at the moment. And it’s hard to predict how long it will take before it’s adopted for consumer and industrial applications since there are many factors that weigh on that renaissance.
For example, is the multi-trillion dollar semiconductor industry willing to retool and change its infrastructure? Mahjouri-Samani believes it will be a gradual change, with new industries born slowly and joining with larger industries.
“Well, It is an evolution” he said.
[i] Sajedeh Manzeli, Dmitry Ovchinnikov, Diego Pasquier, Oleg V. Yazyev & Andras Kis. 2D transition metal dichalcogenides, Nature Reviews Materials. 2017
[ii] Mahjouri-Samani, M., Lin, MW., Wang, K. et al. Patterned arrays of lateral heterojunctions within monolayer two-dimensional semiconductors. Nat Commun 6, 7749 (2015). doi.org/10.1038/ncomms8749
[iii] Zabihollah Ahmadi, Parvin Fathi-Hafshejani, Emre Kayali, Majid Beidaghi and Masoud Mahjouri-Samani. Rapid laser nanomanufacturing and direct patterning of 2D materials on flexible substrates—2DFlex. 2021 Nanotechnology 32 055302
[iv] Nurul Azam, Zabihollah Ahmadi, Baha Yakupoglu, Salah Elafandi, Mengkun Tian, Abdelaziz Boulesbaa and Masoud Mahjouri-Samani. Accelerated synthesis of atomically-thin 2D quantum materials by a novel laser-assisted synthesis technique. 2020 2D Mater. 7 015014
[v] Zabihollah Ahmadi, Baha Yakupoglu, Nurul Azam, Salah Elafandi and Masoud Mahjouri-Samani. Self-limiting laser crystallization and direct writing of 2D materials. Published April 16, 2019. IOP Publishing Ltd on behalf of the IMMT
[vi] Parvin Fathi-Hafshejani Nurul Azam, Lu Wang, Marcelo A. Kuroda, Michael C. Hamilton, Sahar Hasim*, and Masoud Mahjouri-Samani. Two-Dimensional-Material-Based Field-Effect Transistor Biosensor for Detecting COVID-19 Virus (SARS-CoV-2). ACS Nano, June 28, 2021
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