Semiconductors and semimetals, - Applications and devices part A

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In addition to the electrical constraint, the nature of the mechanical constraint on the monolayer is also expected to play a role in the phase boundary, discussed in Supplementary Note 4. The distinction between the constant charge and voltage cases is most important when a phase transformation occurs. A constant charge condition exists when the charge on the monolayer remains constant during the phase transition as if it is electrically isolated. An approximate condition of constant charge could exist when adsorbed atoms or molecules donate charge to the monolayer.

Figure 3a is a schematic of the system. The monolayer TMD is a distance d away from the electron reservoir metal electrode. Because the dielectric medium is vacuum in this schematic, E I and E c in equation 1 can be combined, which can be understood from Fig. The units are per formula unit f. The red line represents the constant-area case, in which the monolayer is clamped to its 2H lattice constants.

The charge thresholds exhibit a significant dependence on the relaxation of lattice constants, indicating that the precise transition point may be sensitive to the presence of a substrate. In both cases, transition at constant stress is more easily induced than the transition at constant area. Variation of the results of Fig. The blue lines are constant-stress stress-free cases, in which both phases exhibit minimum energy lattice constants and atomic positions.

This condition is expected to hold when the monolayer is freely suspended or is not constrained by friction on a substrate. The red lines represent constant-area cases, where the monolayer is clamped to its 2H lattice constants. This condition might be expected to hold when there is a strong frictional interaction between the monolayer and substrate preventing the monolayer from relaxing freely.

Figure 3b shows that semiconducting 2H-MoTe 2 has lower free energy and is the equilibrium state when the monolayer is electrically neutral or minimally charged. In the constant-area case red line in Fig. This suggests that the precise transition point may be sensitive to the presence of a substrate and that the detailed nature of the mechanical constraint of the monolayer may play a substantive role in the magnitude of the phase boundaries.

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Figure 3c shows that the transition in monolayer MoS 2 requires much larger charge density than the MoTe 2 case. This is consistent with previous experimental reports that adsorbed species donating negative charge to monolayer MoS 2 can trigger a trigonal prismatic to octahedral structure transformation 24 , See Supplementary Fig.

Another relevant type of electrical constraint is fixed voltage or electron chemical potential. This constraint is most applicable when the monolayer is in an electrostatic gating structure similar to field-effect transistors made using monolayers. Many distinct electrostatic gating device structures can be utilized to realize this dynamic control through a change in carrier density or electron chemical potential of the monolayer. Here we consider a capacitor structure shown in Fig. A monolayer of MoTe 2 is deposited on top of a dielectric layer of thickness d , which we take to be HfO 2 with a large dielectric constant of 25 ref.

Monolayer and dielectric are sandwiched between two metal plates between which a voltage V is applied. High-dielectric constant material HfO 2 is chosen to increase the capacitance and hence increase the charge density in the monolayer. The metal plate is chosen to be aluminum with a work function of 4. The curves in Fig. We compute the total energy and equilibrium grand potential of this system using equations 1, 2, 3, 4, 5. Voltage V is applied between the monolayer and the aluminum plate. Between the two transition voltages, semiconducting 2H-MoTe 2 has a lower grand potential and is thermodynamically stable.

The red dashed line in c represents a breakdown voltage 35 obtained experimentally for a HfO 2 film of thickness 4. Plotted in Fig. Two transition voltages are labelled also using black dashed lines. The experimental breakdown voltage for a 4. This breakdown field in HfO 2 is larger than some other reports and may depend on the details of growth 36 , Therefore, employing an appropriate dielectric is likely to be critical here in observing the phase change.

Ionic liquids may be employed to help address the challenge of achieving large voltages. Ionic gating has been applied to a variety of TMDs to investigate superconductivity 38 by measuring I—V curves. However, when a large voltage is applied, it may be challenging to probe structural phase transitions from I—V curves alone due to a large density of charge in the TMDs. Structural characterization approaches, such as Raman spectroscopy, may provide a more direct probe of electrically induced structure phase transitions in monolayer TMDs.

While the curves in Fig. These phase diagrams predict the thermodynamically favoured phase as a function of voltage V and thickness d of the HfO 2 dielectric medium. In each phase diagram, there exist two phase boundaries, the positions of which vary with the work function W of the capacitor plate.

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The transition voltages increase with the thickness of the dielectric layer. In analogue with the changes in charge density phase boundaries shown in Fig. If the substrate constrains the area of the monolayer across the transition through friction, the voltages in Fig. The figure also shows a reported experimental breakdown voltage of a 4. In each phase diagram, there exist two phase boundaries that vary with the work function W of the capacitor plate. The required transition gate voltage is smaller in the constant-stress case a than in the constant-area case b. The red triangle represents the breakdown voltage of a 4.

Field-effect transistors based on few-layered MoTe 2 have been reported in ref. Monolayer alloys present the possibility for reducing the required gate voltage by varying the chemical composition. Recently, monolayer alloys of Mo- and W-dichalcogenides have attracted increasing attention for their tunable properties 40 , 41 , 42 , 43 , Therefore, one might expect the energy difference between the two charge-neutral phases to be tunable through zero with alloy composition.

The smaller the energy difference is, the closer the phase boundary is to ambient condition and the smaller the external force required to drive the phase transition in monolayer TMDs Therefore, controlling alloy composition is likely to enable tuning of the transition gate voltage.


However, the precise value of x required to achieve a particular transition voltage is likely to depend on a number of factors including synthesis conditions and mechanical constraints 44 , Detailed cluster expansion calculations for these monolayer alloys are presented in ref. For example, assuming HfO 2 medium of 4. Plotted is the phase stability of a representative alloyed monolayer Mo 0.

The transition is assumed to occur at constant monolayer area. The magnitudes of the transition gate voltages in this alloy are smaller than those of pure MoTe 2 monolayer indicating the potential for phase boundary engineering. One might expect that the transition gate voltage in monolayers can be tuned and reduced potentially arbitrarily by controlling the chemical composition of this and other potentially alloys.

Alternative mechanical constraints placed on the alloy monolayer for example, constant stress can also be expected to shift the phase boundary and transition gate voltage. Electrically induced structural phase changes between 2H and 1T phases in the Ta-based TMDs, TaSe 2 and TaS 2 , have been reported in experiments using a STM tip 17 , 18 , although the mechanism for this reported effect may differ from the charge-induced effect reported in the present work.

As a supplement to the previous calculations on group VI TMDs, we have computed the constant-stress phase diagram of monolayer TaSe 2 in the capacitor gating structure shown in Fig. Figure 7b shows that the phase diagram of TaSe 2 has a phase boundary at only positive gate voltage, qualitatively different from MoTe 2 and MoS 2.

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We find that this difference results from the metallic nature of both 2H- and 1T-TaSe 2 , further discussed in Supplementary Note 5. Unlike MoTe 2 , the phase diagram of TaSe 2 only has one phase boundary, which corresponds to a positive gate voltage. Below the phase boundary, 2H-TaSe 2 is more stable; above the boundary, 1T has lower energy and is more stable. Intuition for this qualitative difference from MoTe 2 is provided in Supplementary Note 5.

A qualitative difference between these calculations and the STM experiment 17 is the observation of the transition at both signs of STM bias, suggesting that other effects could be at play in the experiment. Further quantitative comparison with the experiment is made challenging by the small separation between the monolayer and the STM tip Supplementary Fig. The electrical dynamical control of structural phase in monolayer TMDs has exciting potential applications in ultrathin flexible 2D electronic devices.

If the kinetics of the transformation are suitable, nonvolatile phase-change memory 9 may be an application. One might expect 2D materials to have energy consumption advantages over bulk materials due to their small thickness. If the kinetics is sufficiently fast, another potential application may be subthreshold swing reduction in field-effect transistors to overcome the scaling limit of conventional transistors 4.

In addition, the change in the transmittance of light due to the phase transition of monolayer TMDs may be employed in infrared optical switching devices, such as infrared optical shutters and modulators for cameras, window coating and infrared antennas with tunable resonance. To summarize, we have identified a new mechanism, electrostatic gating, to induce a structural semiconductor-to-semimetal phase transition in monolayer TMDs. We discover that changing carrier density or electron chemical potential in the monolayer can induce a semiconductor-to-semimetal phase transition in monolayer TMDs.

These transition charges and voltages are expected to vary considerably with the nature of the mechanical constraint of the monolayer and also potentially the presence of dopants or Fermi level pinning.

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The convergence thresholds for electronic and ionic relaxations were chosen to be 0. Spin—orbit coupling is employed in all DFT calculations. The ionic relaxations were performed using conjugate gradient algorithm. All calculations in this work were performed at zero ionic temperature, omitting the vibrational component of the free energy.

Therefore, one would expect inclusion of these effects to decrease the magnitude of the transition charge density and gate voltage calculated in this work. See Supplementary Note 7 for a discussion on vacuum electronic states. How to cite this article: Li, Y. Structural semiconductor-to-semimetal phase transition in two-dimensional materials induced by electrostatic gating.

Lencer, D. A map for phase-change materials. Wong, H. Phase change memory. IEEE 98 , — Metal-oxide RRAM. IEEE , — You probably have some of this in your home. If you drove to work or school today, you used silicon. Silicon may have also been used to make the mirrors and windows in your car as well as your home, and a dioxide of silicon may also be scattered across your front yard as sand.

This element is frequently used to change the color of fireworks because it burns green. This might be one of the most explosive uses of metalloids in our daily life, at least until you make it down to tellurium. You probably have some boron in your kitchen or garage, in the form of boric acid or borax.

Pyrex is comprised of borosilicate glass, which combines both boron and silicon to make long-lasting cookware that is heat resistant and will last long enough for you to pass down to your grandchildren. You need to make sure that you get Pyrex dishes from France though. Its primary use is in the semiconductor industry. Like silicon, germanium is mixed with various other elements to improve its conductive properties, so you can most often find it in electronic devices. This element can also be used in fluorescent lamps and infrared detectors.

Semiconductors and Semimetals, Volume 97: Semiconductors for Photocatalysis

Some studies suggest that germanium can be used to kill bacteria, and it may have future applications in chemotherapy treatments. When mixed with gallium, it becomes a semiconductor and is often used in LEDs. If you enjoy hunting or target shooting, you may be using more arsenic than you realize. The lead alloys used to make bullets often contain it. Oddly enough, a small amount of arsenic is necessary for the human body to grow and thrive.

The ancient Egyptians used antimony as a cosmetic, centuries before scientists officially discovered it in In its pure form, it acts as a semiconductor in things like diodes. Once alloyed with other metals, it is used in batteries, cable sheathing and other similar applications. You may also find it in your art class — the Egyptians had the right idea there.

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