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. 2020 Oct 6;117(40):24634-24639.
doi: 10.1073/pnas.2013565117.

Metallization of diamond

Zhe Shi  1   2 , Ming Dao  3 , Evgenii Tsymbalov  4 , Alexander Shapeev  4 , Ju Li  3   2 , Subra Suresh  3   5
Affiliations

Metallization of diamond

Zhe Shi et al. Proc Natl Acad Sci U S A. .

Abstract

Experimental discovery of ultralarge elastic deformation in nanoscale diamond and machine learning of its electronic and phonon structures have created opportunities to address new scientific questions. Can diamond, with an ultrawide bandgap of 5.6 eV, be completely metallized, solely under mechanical strain without phonon instability, so that its electronic bandgap fully vanishes? Through first-principles calculations, finite-element simulations validated by experiments, and neural network learning, we show here that metallization/demetallization as well as indirect-to-direct bandgap transitions can be achieved reversibly in diamond below threshold strain levels for phonon instability. We identify the pathway to metallization within six-dimensional strain space for different sample geometries. We also explore phonon-instability conditions that promote phase transition to graphite. These findings offer opportunities for tailoring properties of diamond via strain engineering for electronic, photonic, and quantum applications.

Keywords: elastic strain engineering; machine learning; materials under extreme conditions; metallic diamond; multiscale simulations.

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Conflict of interest statement

Competing interest statement: Z.S., M.D., J.L., and S.S. are coinventors on a patent application based on the invention reported in this paper.

Figures

Fig. 1.
Fig. 1.
Metallization of diamond. (A) Electronic band structure k-space plot showing complete closure of bandgap leading to metallization of diamond which is subjected to deformation at a 6D strain state of (0.0536, −0.0206, −0.056, 0.0785, 0.0493, 0.0567) in the [100][010][001] coordinate frame. An entire region of strains exists for the metallization of diamond and a 2D cross-section plot of normal strain components ε11 and ε22 is illustrated in B. The axes in B are absolute strain component values of ε11 and ε22, with the other four strain components fixed at −0.056, 0.0785, 0.0493, and 0.0567. Color contours indicate regions of constant elastic strain energy density (h) for different deformation states. The black star symbol denotes the strain energy density value, h = 98.7 meV/Å3, which corresponds to the band structure plot shown in A.
Fig. 2.
Fig. 2.
Stratification of the strain hyperspace into regions of metallization and bandgap transition in diamond. (A) Metallization in elastically strained diamond for different values of normal strain components ε11, ε22, and ε33, with the other three strain components held fixed. The plane with ε33=−0.056 (colored as light green) cuts the 3D volume and results in a projection onto the ε11–ε22 2D plane. (B) Detailed characterization of the ε11–ε22 strain space includes a region of direct metal (brown) strains within the region of direct bandgap (blue) strains and a region of indirect metal (brown) strains within the nonzero indirect bandgap strains (white zone with magenta symbols). The black star in indicates the same strain case (0.0536, −0.0206, −0.056, 0.0785, 0.0493, 0.0567) discussed in Fig. 1. An alternative visualization of the metallization strains in A is presented in SI Appendix, Fig. S2. (C) GW band structure of the diamond strained within the “safe” metallization region resulting in an indirect metal. Strained diamond (D) with a direct bandgap (point d in B) and (E) with an indirect bandgap (point e in B). The strain region of phase transformation in diamond (usually associated with phonon instability) is shaded in gray in B. (F) A phonon density of states (DOS) plot corresponding to point f in B illustrates imaginary phonon frequencies (indicated by the magenta arrow) when structural instability occurs. (Inset) A magnified view near zero frequency.
Fig. 3.
Fig. 3.
Metallization in diamond nanoneedles. (A) Schematic of the bending of single-crystalline diamond nanoneedle by diamond nanoindenter tip inside a scanning electron microscope. (B) FEM predictions of the local compressive and tensile strain distributions (left and middle needle, respectively) and predictions by the machine-learning algorithm of the distribution of bandgap (right needle) for a diamond nanoneedle with its <110> crystallographic direction aligned with the needle axis. (Inset) A scanning electron micrograph of the deformed nanoneedle during the bending experiment, from ref. . Reprinted with permission from AAAS. (C) Increasing magnitude of bending in the <110> nanoneedle causes a significant reduction in bandgap of diamond from 5.6 eV (zero strain) down to 0 eV for a maximum local compressive strain of −10.8% (the corresponding maximum local tensile strain on the tension side is 9.6%). (D) Local tensile strain beyond 12.1% results in fracture or graphitization on the tensile side of the nanoneedle according to our ab initio calculations, even when there are no preexisting defects. See also Movie S1 for the evolution of elastic strain energy, bandgap, and the corresponding band structure at the maximum compression site in the nanoneedle, showing the medialization process.
See this image and copyright information in PMC

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