Nanoscale Engineering in VO2 Nanowires via Direct Electron Writing

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Nanoscale Engineering in VO2 nanowires via Direct Electron Writing Process Zhenhua Zhang, Hua Guo, Wenqiang Ding, Bin Zhang, Yue Lu, Xiaoxing Ke, Weiwei Liu, Fu-Rong Chen, and Manling Sui Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04118 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 12, 2017

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Nanoscale Engineering in VO2 nanowires via Direct Electron Writing Process Zhenhua Zhang1, Hua Guo2, Wenqiang Ding1, Bin Zhang1, Yue Lu1, Xiaoxing Ke1, Weiwei Liu4, Furong Chen3*, & Manling Sui1* 1

Institute of Microstructure and Properties of Advanced Materials, Beijing University of Technology, Beijing 100124, China

2

Department of Materials Science and NanoEngineering, Rice University, Houston, TX 77005, USA 3

Department of Engineering and System Science, National Tsing Hua University, Hsin Chu 300, Taiwan 4

Beijing Computational Science Research Center, Beijing 100084, China

ABSTRACT: Controlling phase transition in functional materials at nano-scale is not only of broad scientific interest, but also important for practical applications in the fields of renewable energy, information storage, transducer, sensor, etc. As a model functional material, vanadium dioxide (VO2) has its metal-insulator transition (MIT) usually at a sharp temperature around 68 °C. Here we report a focused electron beam can directly lower down the transition temperature of a nano-area to room temperature without pre-patterning the VO2. This novel process is called radiolysisassisted MIT (R-MIT). The electron beam irradiation fabricates a unique gradual MIT zone to several times of the beam size, in which the temperature-dependent phase transition is achieved in an extended temperature range. The gradual transformation zone offers precisely controlling the ratio of metal/insulator phases. This direct electron writing technique can open up an opportunity to precisely engineer nanodomains of diversified electronic properties in functional material-based devices.

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KEYWORD: Vanadium dioxide (VO2), metal-insulator transition (MIT), transition temperature, radiolysis, oxygen vacancy, domain wall, Vanadium dioxide (VO2), a strongly correlated electron material, has attracted intensive interest in its metal-insulator transition (MIT)1-4, which gives rise to a 3−6 orders of magnitude difference in conductivity5 and a significant change in the optical properties6 at about Tc (~68 °C). The MIT can be triggered by external stimuli, such as heating, straining7-10, impurity doping11-16, and an electrical field17, 18. Although the driving mechanism of MIT is not completely revealed6,

19-24

, the strong electron

correlation (Mott–Hubbard transition)20 and/or electron–lattice coupling (Peierls transition)19 are generally thought to be responsible for the transformation. For thermally driven transition, with temperature above the Tc, the rutile (R) phase behaves as a half-filled metal with a 3d1 state that 3d electrons delocalize around V+4, while below the Tc, a transformation to insulator (monoclinic, M) phase takes place so that dimerized V+4 pair ions is tilted with respect to the c-axis direction, causing 3d electrons to localize around V sites and forming the splitting of energy band19. Since tilted dimerized V+4 pairs can induce the expansion of ~1% along the tetragonal caxis, MIT can also be triggered by applying local uniaxial compressive stress along equivalent c axis in M phase8, 10. With impurity doping, MIT can be achieved, for instance, by injection of W+6 to form electronic defects of V+3-V+4 and V+3-W+6 pairs that donate delocalized electron in VO2 which obviously lowers the MIT temperature. As for the electric field effect, MIT can be induced by high electric field18 of about 6.5×107 V/m, and the metallic phase can be stabilized via an electrolyte gating process25, 26 that induces extra oxygen vacancies. The oxygen vacancy is an electron donor that plays the same role as W+6 dopant, which favors formation of V+3-V+4 pairs, thus promoting the stabilization of metallic phase. For the potential applications


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at the nanoscale, there were many reports of the MIT on pre-defined nano-areas such as the nano-patterned film27,

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, nano-puddle21 and nanowire8,

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. Here, we

demonstrate that electron beam itself can act as a nano-probe to directly trigger the MIT at nanometer scale at room temperature without any constrained geometry. This process is called radiolysis-assisted MIT (R-MIT). The single-crystalline VO2 nanowires employed in this study were grown directly on oxidized silicon wafers by a physical vapor transport technique at 900 °C to 1000 °C following Hongkun Park’s method30. The in situ heating experiments were performed on a FEI G2 20 TEM (200 kV) equipped with Gatan commercial double tilting heating holder, Gatan Model 652, with a temperature accuracy of 0.1 °C. The temperature output values were calibrated by a standard sample VO2. To ensure that the sample temperature was consistent with that of the measured temperature, we waited for at least 15 minutes to achieve thermal equilibrium before further imaging. The VO2 nanowires were slightly scraped onto copper substrate or transferred to copper substrate by using nano-manipulator equipped in FEI Helios NanoLab 660 Focused Ion Beam (FIB), to build the nanowire cantilever. The current-voltage (I-V) curves were measured by a STM-TEM probing system (Nanofactory). Electron Energy Loss Spectrum (EELS) line scan was acquired by the Gatan Imaging Filter (GIF) system with a 1.1 eV energy resolution installed in FEI Titan G2 80-200 ChemiSTEM (200 kV). The dose rate in our experiment is always set to be of about 2×105 e-nm-2s-1. The total dose is controlled by the accumulated irradiation time. Figure 1a shows the conventional abrupt MIT when the temperature was raised up to 68 °C in a pristine VO2 nanowire. In contrast, after irradiation at the free end of the same nanowire with a focused electron beam of about 200 nm in diameter for 6


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minutes, Figures 1b and 1c show a gradual transitional process with reduction of the critical temperature upon heating/cooling for the R-MIT. At the beginning, the R phase was found to preferentially nucleate just at the irradiated free end about 200 nm in size when heating up to 62.2 °C (Figure 1b). As temperature increased up to 67.2 °C, the R phase expanded gradually up to a size of about 1 µm with consumption of M1 phase. The temperature-dependent area within 1 µm is termed gradual transformation zone (GTZ), since the phase transition in this zone can take place in an extended temperature range and therefore the M1/R phase ratio can be well controlled during the heating process. Beyond this point, the residual M1 phase transformed into R phase suddenly at 68 °C. In the cooling process (Figure 1c), the R phase transformed back to M1 phase rapidly and stopped at the same site about 1 µm away from the free end at 65.2 °C. Again, the R/M1 domain wall in the GTZ was controllable to migrate gradually toward the irradiated site as temperature decreased. Finally, the R phase was converted completely back to M1 phase at 58.7 °C. It is known that the energetic electron beam interacts with the transition metal oxide, and desorption of the oxygen atoms takes place via radiolysis of core-hole Auger decaying process31. Basically, the electron stimulated desorption process involves inelastic scattering of the incident electrons with the inner shell electrons of oxygen ion, which leads to a neutral (or positive) oxygen atom which is repelled by the surrounding metal ions and ejected into the vacuum leaving an oxygen vacancy around surface. As a result, oxygen vacancy dopants lower the Tc as W+6 dopants that favor formation of V+3-V+4 pairs. The existence of V+3 is confirmed with chemical shift in electron energy loss spectra (EELS)32, 33 (Figure 2 and supporting information S1). The oxygen vacancy doping by electron beam is thus a direct electron doping process (see supporting information S2). At the same time, an electric field is built up


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mainly along the VO2 nanowire due to emission of the secondary and Auger electrons into the vacuum34 which results in the accumulation of positive charges in the irradiated area. This happens especially when the dose rate is high as in our case that the electric field cannot be compensated quickly with the grounded electrode. Such self-built electric field can be measured using off-axis electron holography and the model was further improved and demonstrated quantitatively recently35. The maximum strength of self-built electric field is estimated to be 140 V/m in our experiment (supporting information S3). The self-built electric field may drive the positive charged oxygen vacancies around surface diffusing radially inward to the bulk of nanowire as well as migrating away from the irradiated area that causes spreading of the oxygen vacancy distribution to be around 1 µm from the free end in this case (supporting information S4 and S5). As expected, the reduction of MIT temperature in the spreading zone is associated with the oxygen vacancy dopants. It is consistent with the temperature-dependent GTZ observed in Figures 1b and 1c during the heating/cooling processes, which is clearly distinguished from the abrupt transition zone in the same nanowire. A systematic study of the equivalent E-field effect on controlling GTZ in a device structure VO2 is an undergoing experiment, which is beyond the scope of the current paper. With a brief summary, the R-MIT process in the current experimental condition allows to directly fabricate well controllable GTZs of a size about five times of the beam size. The behavior of R-MIT can be controlled cooperatively by irradiation dosage as well as the inelastic scattering cross-section σ of incident electron in oxygen atom which depends on the energy of incident beam. The dependence of R-MIT on the beam energy is discussed in detail in the supporting information S6. Figure 3a shows the Tc decreases with increment of electron dosage for an individual nanowire. A


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minimum critical dosage (7.2×107 nm-2 in this case) is required to fabricate the controllable GTZ for the R-MIT, otherwise the conventional MIT with a sharp Tc is observed. The inset in Figure 3a shows that the the Tc can be tuned down to the room temperature over a wide range of electron dose. Figure 3b demonstrates that both Tc and the size of GTZ can be tuned by electron doses, from 8.5×107 nm-2 to 1.2×108 nm2

. Figure 3c shows the correlation between the Tc reduction and the diameter of

nanowires under the same dose. The thinner nanowire is more sensitive to the electron irradiation. Basically, higher dose induces higher concentration of oxygen vacancy in VO2 that gives a lower Tc and a wider GTZ. As the dose increases to 5.6×109 nm-2, the Tc can be lowered to even room temperature. It is estimated from the Figure S1 that the equivalent concentration of the oxygen vacancy is roughly 10% for the dose of ~1010 nm-2, while in an earlier report36, the Tc of VO1.96 powder is reduced from 68 °C to 50 °C with estimated oxygen vacancy of 1.3% from its stoichiometry, which is consistent with the trend given in Figure 3a. A series of electron diffraction patterns (Figure 3d) were recorded at different amount of total doses from 0.7×109 to 5.6×109 nm-2. The diffraction intensities in Figure 3e were averaged from the same lattice family of diffraction spots for the corresponding total doses, respectively. The diffraction spots {100}M start dimming at the dose of 0.7×109 nm-2 and finally vanish at the dose of 5.6×109 nm-2 (Figure 3e, right). On other hands, the superimposed spots {011}M∥{110}R become brighter as the dose accumulates (Figure 3e, left). This implicitly indicates that the initial M1 phase is continuously metalized toward R phase even at room temperature above a critical dose of 0.7×109 nm-2. At the higher accumulated dose of 5.6×109 nm-2, the irradiated tip entirely turns into R phase at room temperature (Figure 3d, right).


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Figure 4a shows the good reproducibility of R-MIT for several heating/cooling cycles and even after the nanowire is kept one day outside of TEM in atmospheric air at room temperature (Figure 4a). Both the GTZ size and the temperature dependence of the R/M domain within the GTZ do not change with the heating/cooling history below 100 °C and the vacuum/atmospheric environment. However, the R-MIT can be controllably recovered by annealing in oxygen atmosphere at higher temperature (for example at 250 °C) to erase the oxygen vacancy doping (see supporting information S7), where the Tc was raised while the GTZ was reduced, as shown in Figure 4b (also see Figure S6). The superior characteristic of the R-MIT as compared with the traditional MIT is that the slow motion of the R/M domain wall within the GTZ allows one to tune precisely the electronic properties of the functional material. Figure 4c shows the outstanding manufacturability of R-MIT for a four functional domains in a single VO2 nanowire using a 200 nm electron beam with a dose ~108 nm-2. The adjacent R domains are only about 500 nm apart, which indicates the potential of nanoscale manufacturability of our method. It is reasonable to envisage that the functional domains in VO2 can be controlled down to sub-nanometers in size with a sub-Å electron probe in an aberration corrected optical system. In summary, the transition temperature of MIT in VO2 nanowires was successfully lowered down to even room temperature by electron radiolysis process. Oxygen vacancy generated in the radiolysis process plays a role as an electronic dopant that offers extra electron carriers, which is responsible for reduction of the transition temperature. Unlike the conventional MIT taking place at a sharp Tc, the RMIT can be manipulated in an extended transition temperature range via introducing


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the GTZ, which allows one to precisely control the ratio of insulator/metallic phases. The R-MIT can be achieved at a nanometer scale that may open up opportunities for fabricating nano-sized functional domains in functional material-based devices.

Figure 1. Conventional MIT vs. Radiolysis-assisted MIT. a, Instant phase transformation (conventional MIT) in a VO2 nanowire at to 68 °C via heating (M1 phase, upper and R phase, down). Scale bar is 1 µm. Diffraction patterns confirm the monoclinic and rutile structures for the M1 and R phases, respectively. The VO2 single crystal nanowire has a length of 3.7 µm, a cross sectional area of 7.1×10-2 µm2, with one end attached on a copper substrate. b and c, Gradual phase transformation (R-MIT) during heating (b) and cooling (c) after the free-end of nanowire is irradiated with electron beam. Scale bar is 1 µm. The positions of M1/R domain wall are indicated with short black lines. A gradual transformation zone (GTZ) was found about 1 µm away from the free end in the heating/cooling processes.


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Figure 2. Chemical shift analysis of V+3. a, EELS line scan across non-irradiated (position #1) and irradiated areas (position #2 and #3, with an electron dose of 1.6×109 nm-2). b, EELS profile at the position #1, #2 and #3. Peak positions of V-L3, L2 edges as well as O-K energy loss near edge structure (ELNES) P1, P2 pre-edges for VO2 are marked as referenced. Chemical shifts are evident at positions #2 and #3.


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Figure 3. The dose dependent R-MIT. a, The Tc decreases with increase of dose. Above a critical dose of 7.2×107 nm-2 (blue area), the GTZ is visible for the nanowire with a diameter of 270 nm. An inset is shown with an additional data point from the Figure 3d that Tc can be lowered down to the room temperature (25 ºC, yellow square) with a sufficiently high dose (560×107 nm-2). It is noted that the horizontal axis is given with a logarithmic scale of dose to cover the effect of dose on the Tc over a wider range. Since this additional data point is from another nanowire with a different diameter (140 nm in diameter), this data point is highlighted with yellow square. b, Temperature vs. the transformation fraction (length of transformed R phase/total length of nanowire) for different doses of 8.5×107 nm-2 (blue), 9.6 ×107 nm-2 (red), 1.2×108 nm-2 (green) in the corresponding GTZs (marked by dashed lines), respectively. The Tc (at fraction ~0) decreases and the size of GTZ increases with increment of dose. c. A correlation between the Tc reduction and the diameter of


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nanowires under the same dose of 108 nm-2. The Tc is lower for the thinner nanowire. d. Electron diffraction patterns of a VO2 nanowire at different irradiation doses (from left to right, 0, 0.7×109, 1.6×109, 5.6×109 nm-2). e, Intensities of the diffraction spots versus the irradiation doses. Left: {011}M∥{110}R superimposed spot for both M and R phases. Right: {100}M spot for M phase alone.

Figure 4. Reproducibility and manufacturability of R-MIT process. a, Fraction of transformation (R phase) of a pre-irradiated VO2 nanowire for three heating cycles. Shadow area shows a consistent GTZ for these three cycles. Nanowire was kept in air at room temperature for one day before the 3rd heating. b, Recovery of R-MIT by annealing in oxygen atmosphere and the transition temperature, Tc, and the size of GTZ during the MIT of the same nanowire in pristine, irradiated and annealed state are plotted. c. Four functional domains are patterned in a single VO2 nanowire by swinging electron probe across nanowire for multi-step doping of oxygen vacancies.


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The electron beam has a size of 200 nm in diameter with a dose of ~108 nm-2. Scale bar is 300 nm. ASSOCIATED CONTENT Supporting Information Additional information on quantification of oxygen vacancy concentration by EELS, I-V curves on irradiation, estimation of self-built electric field and migration of oxygen vacancy, DFT calculation of diffusion energy barrier of oxygen vacancy, calculated inelastic scattering cross section vs electron beam energy, and recovery of R-MIT by annealing, is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *(M. L. Sui) Email: [email protected] *(F. R. Chen) Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge the support by the National Natural Science Foundation of China (Grants Nos. 11374028 and U1330112). M.L.S. acknowledges the Cheung Kong Scholars Programme of China. F.R.C acknowledges the support from NSC 962628-E-007-017-MY3 and NSC 101-2120-M-007-012-CC1. The authors acknowledge very useful discussion with Li-Min Liu. The authors also thank Jin-Hua Hong for help with EELS measurement. We acknowledge the computational support from a Tianhe-2JK computing time award at the Beijing Computational Science


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Research Center (CSRC) and the Special Program for Applied Research on Super Computation of the NSFC- Guangdong Joint Fund (the second phase).

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