Axial Modulation of MetalâInsulator Phase Transition of VO2

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Axially Modulation of Metal-Insulator Phase Transition of VO Nanowires by Graded Doping Engineering for Optically Readable Thermometers Run Shi, Jingwei Wang, Xiangbin Cai, Linfei Zhang, Pengcheng Chen, Shiyuan Liu, Liang Zhang, Wenkai Ouyang, Ning Wang, and Chun Cheng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08946 • Publication Date (Web): 20 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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The Journal of Physical Chemistry

Axially Modulation of Metal-Insulator Phase Transition of VO2 Nanowires by Graded Doping Engineering for Optically Readable Thermometers Run Shi,a,b Jingwei Wang,a,b Xiangbin Cai,b Linfei Zhang,a Pengcheng Chen,a Shiyuan Liu,a Liang Zhang,a Wenkai Ouyang,a Ning Wangb and Chun Chenga* a

Department of Materials Science and Engineering, Southern University of Science

and Technology, Shenzhen 518055, P. R. China b

Department of Physics and Center for 1D/2D Quantum Materials, the Hong Kong

University of Science and Technology, Hong Kong, P. R. China

* To whom correspondence should be addressed, electronic mail: [email protected]

ABSTRACT: Temperature measurement is critical for many scientific experiments and technological applications. Diverse thermometers have been developed for the thermal sensing at macroscopic length scales. However, in situ and quantitative temperature measurement of nanoscale objects in a convenient approach is still a challenge. Here, we demonstrate a new type of optically-readable VO2 nanowire-based thermometer, basing on the unique axially-gradient phase transition behavior along single-domain VO2 nanowires, which is attributed to the hydrogen

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doping of single-crystalline VO2 nanowires through hydrothermal fabrication and the hydrogen engineering via a post-annealing process. Besides the appropriate microscopic size and user-friendly operation, the as-prepared optically-readable VO2 nanowire-based thermometers have ultra-high relative sensitivity(~17.4%/K) and temperature resolution(~0.026K), enabling the sensitive monitoring of the thermal environment of small spaces or the temperature of even nanoscale structures.

Introduction The fast and active development of nano science and technology increases the urgent demands for developing more accurate quantitative measurement techniques of low-dimensional interactions. Up to now, robust solutions for in situ and quantitative measurement of local temperature at tiny, nanoscale objects are still limited. Typical small-scale thermal sensing techniques such as infrared/luminescence-based thermometers,1-4 thermal expansion-based thermometers,5,6 micro-thermocouples7-9 and micromechanical resonators10 either suffer from time and cost-consuming sensing unit/chip fabrication, or highly require complicated advanced setups. Hereof, the development of a low-cost, convenient and accurate temperature measurement available at micro/nano scale is essential especially in the emerging thermosensing studies on microelectronics, cellular signaling, nanomedicine, microfluidics, etc.

Vanadium oxides have been widely used in many aspects of energy-related applications for its abundant valence states and complex crystal polymorphs.11-14


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Among them, vanadium dioxide (VO2) attracts increasing attention for its excellent near-room temperature metal–insulator transition (MIT) at around 68 °C and related unique physical phenomena.15,16 Recently, we developed optically readable near-field powermeters based on the MIT in single-crystal VO2 micro/nanobeams which enables the direct quantification of heat flow at one-dimensional systems.17-19 In these studies, a laser beam focusing on the VO2 beam was applied to introduce an axial temperature gradient and thus created a M/I domain wall, which indicates the phase transition temperature (Tc) of VO2 and can be distinguished via optical microscopes owing to the clear optical contrast between the two M/I phases. However, the abrupt MIT of VO2 can only allow single-point or single-temperature detection and thus it is proposed that if we can realize a gradient Tc distribution along the VO2 nanobeams by phase transition engineering, a kind of fascinating optically readable micro/nano thermometers in response to local temperature variation can be achievable, akin to a mercury thermometer. According to the existing publications,20-22 finite size works efficiently in tuning the Tc of VO2 nanocrystals accompanied with a relatively large hysteresis. However, the size-effect might be neglectable in the study of sub-micron/micron VO2 crystals and extremely tiny structure is not friendly to observation. In addition, strain also plays an important role in the modulation of the MIT behavior of VO2 nanocrystals by stabilizing the rutile phase or metastable monoclinic phases at a relatively low temperature.23-25 However, the strain engineering is relatively unmanageable and can be only applied as a supplementary adjustment approach. By contrast, doping is a


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more reliable method to modulate the MIT behavior of VO2. Transitional metal ions (W,20,26-30 Mo,28,29,31,32 Sn,28,29 Sb,29,33 etc.) can be used to adjust Tc by substituting vanadium ions (V4+) to form new cation pairs, which can change the density of carriers along the original V4+-V4+ chains,34 but the typical substitutional-doping methods always cause irreversible damage to the VO2 crystal lattice. As a result, such adjustments are just monodirectional and usually lead to a dramatic super-cooling upon VO2 MIT. Similarly, hydrogen-doping35-37 (H-doping) can also effectively depress the Tc by increasing the electron density in VO2 lattices. Compared to the typical substitutional doping, H-doping is more powerful for its bidirectional adjustment in Tc by the readily achievable hydrogenation and deoxidation reactions, but still suffers from the pronounced hysteresis, which is caused by the asymmetric phase transitions during cooling and heating cycles. Based on the well-developed chemical vapor deposition (CVD) techniques38-41 to grow single-crystalline VO2 nanowires, Lee et al.30 reported a concise approach to produce graded W-doping VO2 nanowires, which can greatly broaden the responding temperature range of VO2 and meet the basic requirements of nanowire-based thermometers for its axial single-domain movement along the nanowires during the gradual MIT behavior. However, CVD growth of graded-doping VO2 is much limited by its low yield, high cost, poor uniformity, weak repeatibility and the as-mentioned irreversible adjustment. In contrast, hydrothermal synthesis is a more promising way to massively produce VO2 (M) nanowires with better reproducibility and various efforts on hydrothermal approaches have been reported.20,42-44 Unfortunately, it is very


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hard to acquire satisfying 1-D VO2 (M) crystals via most of existing hydrothermal methods, which usually suffer from limited size, poor crystallinity, metastable impurity-VO2 (A/B) or uncontrollable MIT. Here, we report a two-step hydrothermal process to fabricate pure VO2 (M/R) nanowires with various H-doping concentrations. It is found that H-doping can facilitate the formation of pure VO2 (M/R) crystals in the hydrothermal reactions and simultaneously influence the corresponding MITs. The phase transition engineering of axially gradient Tc distribution along VO2 nanobeams was realized via simple and effective annealing methods. The domain walls of the annealed VO2 (M) nanowires move free of kinetic obstruction with ambient temperature and therefore they have been demonstrated for optically-readable thermal sensing with ultrahigh temperature resolution and sensitivity.

Methods Hydrothermal Synthesis of VO2 (M) nanowires. In a typical reaction routine, V2O5 (AR, Shanghai Chemical, P. R. China) and H2C2O4·2H2O (AR, Shanghai Hushi, P. R. China) were uniformly mixed at a mole ratio of 1:2 in 80 mL 0.05mol/L H2SO4 (AR, Dongjiang, P. R. China) aqueous solution. After a strong stir, the as-obtained bright orange suspension was transferred into a 100 mL Teflon container and then sealed in a stainless autoclave. After a heat preservation at 100 °C for 10 h, a dark blue precursor solution was obtained. The precursor solution was placed in a high-density poly (para-phenylene) container at a filling ratio of 0.5-0.8 and then sealed into a stainless


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autoclave. After a heating treatment at 260 °C for 24 h and a following natural cooling to room temperature, the precipitates in the container were collected by centrifugation. The final products were washed with deionized water, alcohol and acetone for several times, respectively, and then dried in vacuum at 60 °C for 10 h. Sample A, B and C were prepared under almost the same reaction conditions but at different filling ratios (0.6, 0.67 and 0.73, respectively).

Annealing treatment. The annealing treatments of as-prepared samples were operated in N2 or air gas flow at 200-500 °C for 5-25 min on a METTLER Differential Scanning Calorimeter (METTLER TOLEDO DSC1). Both the heating rate and cooling rate in all the thermal treatments are set as 30 °C/min.

Characterization. The morphology of as-synthesized nanowires was examined by a TESCAN scanning electron microscope (SEM, VEGA 3LMH) and the optical images were taken by an Olympus optical microscope (BX51). The phases of samples were determined by X-ray diffraction (XRD) on a D8 ADVANCE ECO (Bruker) X-ray diffractometer, where the wavelength of generated X-ray was 1.5418 Å (Cu Kα, isolated with a Ni foil filter). The working voltage and current of the X-ray diffractometer were 40kV and 25 mA, respectively. The phase transitions of products were studied by a Differential Scanning Calorimeter (METTLER TOLEDO DSC1), and the measurement temperature range is from -40 °C to 80 °C. A JEOL 2010F instrument operated at 200 keV was used to obtain the high-resolution transmission microscopy (HR-TEM) images and selected area electron diffraction (SAED) patterns.


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The Raman spectra of as-prepared nanowires were obtained by using a HORIBA Raman spectrometer (LabRAM HR Evolution), where the excitation wavelength is 532 nm.

Results and Discussion SEM and TEM are used to demonstrate the typical morphology and crystal structure of as-grown products, respectively. As shown in Figure 1a, the nanowires of sample B have an average length of ~25 µm and diameter ranging from 400 nm to 500 nm. Thus a large aspect-ratio up to 60 can be achieved. Interestingly, the cross section of the VO2 nanowires (inset in Figure 1a) is nearly a round shape, which is different from the rectangular shape of the VO2 nanowires formed by CVD methods.30-33 In addition, the TEM images and SAED pattern shown in Figure 1b, Figure 1c and Figure 1d indicate the as-prepared VO2 nanowire a single-crystalline monoclinic structure with the growth direction along [100]. Careful SAED investigation along the nanowire shows that the diffraction pattern did not change and the diffraction spots were clear and round, suggesting the whole nanowire is free of stacking faults. It is well-known that the controllable growth of pure VO2 (M/R) by hydrothermal methods is a great challenge owing to the complex growth mechanism and abundant by-products.20,42-44 Here, it is discovered that by tuning certain crucial reaction conditions, such as pH and filling ratio, VO2 nanowires with different aspect ratios (Figure S1) can be easily produced. Furthermore, a relatively lower pH (1.20)


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and smaller filling ratio (0.63) can be concluded for the growth of longer VO2 nanowires (90 µm) with larger aspect-ratio (60), and vice versa. We believe that the nucleation density plays a key role in the size control of VO2 nanowires, since both lower pH and smaller filling ratio can hamper the formation of nucleation sites of VO2 in the hydrothermal systems (SI, section S1), allowing the growth of longer nanowires with limited diameter and low yield.

Figure 1. Morphology and crystal structure of as-prepared VO2 (M) nanowires by hydrothermal reactions. (a) SEM image of uniform as-grown nanowires (sample B), where the lower scale bar is 20 µm and the upper scale bar in the inset is 1 µm. (b) TEM image (the scale bar is 500 nm), (c) HRTEM image (the scale bar is 3 nm) and (d) the corresponding SAED pattern (the scale bar is 5 1/nm) of a single VO2


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nanowire (sample B). (e) XRD patterns of sample A, B and C, which were prepared under almost the same reaction conditions but at different filling ratios (0.6, 0.67 and 0.73, respectively). The primary peaks of VO2 (A) (JCPDS 70-2716) in sample A are marked by red triangles. (f) A magnified version of (e) in the 26.5 °≤ 2θ≤ 28.5 ° range.

The existence of undesirable metastable phases is always a big problem in the hydrothermal synthesis of VO2 (M) nanostructures.20,45 XRD analysis on as-prepared products is used to investigate the key reaction factors that promote the growth of pure VO2 (M/R) phase. The XRD patterns shown in Figure 1e indicate that sample A, which was prepared at a relatively low filling ratio of 0.6, contains two components, including main product VO2 (M) (JCPDS 43-1051) and by-product VO2 (A) (JCPDS 70-2716). However, the diffraction peaks of VO2 (A) cannot be found in sample B, which was prepared at a higher filling ratio of 0.67. In the meanwhile, the increasing filling ratio can also contribute to a slight shift of the strongest diffraction peak of as-prepared VO2 samples as shown in Figure 1f, indicating the co-existence of VO2 (R) (JCPDS 44-0253) and VO2 (M) in sample B. The co-existence of M phase and R phase in single nanowires of sample B can be observed in the optical images taken at room temperature (Figure S2). The XPS data (Figure S3) shows that there are only vanadium and oxygen elements in products and no other heavy ions (lithium and above) was detected within the limit of the XPS. Therefore, it is reasonable to deduce that an unintentional H-doping happens during the hydrothermal process and its concentration increases with the filling ratio. Considering a high pressure and acidic


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reaction environment, it is believed that the high concentration of H+ facilitates the hydrogen incorporation into VO2 (M) crystal lattice and thus promotes the stability of the metallic VO2(R) at room temperature.36 Consequently, when further increasing the filling ratio up to 0.73 or more, the corresponding final product (sample C) becomes a hydric vanadium oxide, VO1.75(OH)0.25 (JCPDS 37-0503), which has an orthorhombic structure analogous to the rutile structure of VO2 (R). In addition, it is discovered that increasing the concentration of reductive agents in the precursor solution works like enhancing the filling ratio on that it can also help the elimination of VO2 (A) in VO2 (M/R) and promote the formation of VO1.75(OH)0.25 (Figure S4). Therefore, a rational phase control of as-fabricated products can be easily achieved by tuning the filling ratio or the concentration of reductive agents in the hydrothermal reactions.

DSC is used to investigate the overall MIT behavior of as-prepared samples, assisted by optical microscopy observation on single nanowires. Figure 2a shows the DSC curves of lightly-doped VO2 (sample A), heavily-doped VO2 (sample B) and VO1.75(OH)0.25 (sample C), respectively. A typical DSC curve of VO2 (M) always has two sharp peaks (exothermic and exothermic peaks) at heating and cooling cycles, respectively, as well as a possible hysteresis. Different from the abrupt first-order phase transition happening at 68 °C in un-doped VO2 (M), sample A demonstrates abnormally wide responding temperature ranges in both M→R phase transition upon heating and R→M phase transition upon cooling. This result suggests an uneven H-doping concentration, which is also revealed by the optical observation (Figure 2b and Figure 2c) and Raman analysis of single nanowire (Figure 2d). As shown in


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Figure 2b, with the increase in temperature, M phase nucleates from several sites along the nanowires and the two longest M phase domains grow from both of the tips. Such a domain behavior was observed from almost all the nanowire from sample A and thus it confirms a roughly axially centrosymmetric graded H-doping on single nanowires. Interestingly, the DSC trace of sample B presents two pairs of exothermic (peak 1 and 3) and endothermic (peak 2 and 4) peaks. Compared with sample A, sample B has a higher H-doping concentration and R phase can stably exist at room temperature as shown in Figure 2c, which is consistent with the XRD results (Figure 1f). Therefore, the peaks of 1 and 2 can be attributed to the heavy H-doping, which causes the weakness and the shift of both exothermic and exothermic peaks toward low temperature and a larger hysteresis width than that of sample B. For the new peaks of 3 and 4, two possible mechanisms can be used to explain this unique phenomenon. Firstly, dispersed size distribution31 was believed to contribute to the splitting in the DSC traces of Mo-doping VO2 nanowires. But the as-prepared nanowire sample does not show large polydispersity in size, and the sub-micron diameter weakens the finite size effect on the MIT adjustment of VO2 crystals. Secondly, the randomly distributed vacancies formed accompanying with the crystal growth is more reasonable, which is supported by multi-site R phase nucleation along single VO2 nanowires with the increasing temperature as observed via optical microscopy (Figure 2c). According to the reported work,30,46 the Tc of VO2 is not sensitive to vacancy defects while the presence of vacancies can help the elimination of hysteresis by working as nucleation sites for the growth of M phase domain or


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providing inner stress upon cooling, which can efficiently weaken supercooling. Therefore, we deduce that the existence of peaks 3 and 4, which demonstrates a narrow hysteresis width, can be attributed to the local hydrogen incorporation at the sites of the vacancies in the crystal lattice. In conclusion, the specific MIT behavior of sample B can be ascribed to two factors: the uneven heavy H-doping along the nanowires and the local light H-doping at the attendant vacancies. In addition, the formation of axially gradient hydrogen distribution along single nanowires of sample A & B can be explained by the increasing concentration of hydrogen ions during the hydrothermal growth of VO2 nanowires (SI, section S5). Moreover, as revealed by DSC in Figure 2a, VO1.75(OH)0.25 hardly undergoes a phase transition within the measurement temperature range. The extremely high concentration of hydrogen can make the VO1.75(OH)0.25 crystal lattice stable in a wide temperature range.


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Figure 2. Phase transition behavior of as-prepared samples with different H-doping concentrations. (a) DSC curves of the sample A-C. Optical images of single VO2 nanowires of (b) sample A and (c) sample B during heating. Notably, there is a slight bending in the heavily-doped nanowire during MIT shown in Figure 2c, which is attributed to the radially asymmetric shrink of R domains along cR axis.46 (d) The corresponding Raman spectrum for M phase (yellow parts of the nanowires in Figure 2b and Figure 2c) and R phase (green parts of the nanowires in Figure 2b and Figure 2c).

In this work, the conversion from VO1.75 (OH)0.25 to VO2 and its MIT behavior modification was achieved by an annealing process in N2 gas flow. As shown in the Figure 3a and Figure 3b, with the increase of annealing temperature within a proper range (300-400 °C), both the phase transition temperature upon heating (Tc-h) and upon cooling (Tc-c) increases while the corresponding hysteresis width decreases. It is found that an annealing temperature lower than 280 °C did not bring any change to MIT due to a high energy barrier for the escape of hydrogen, while an excessivelyhigh annealing temperature (>400 °C) resulted in a poorer phase transition behavior owing to the heat-driven deoxidization process in an oxygen-deficient condition.47 Therefore, the annealing temperature range of 300-350 °C is preferable in this case. In addition, an increasing annealing time also leads to the increase of both Tc-h and Tc-c as well as a decrease in hysteresis width as shown in Figure 3c and Figure 3d. However, it is


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discovered that an annealing time longer than 20 min did not further affect the MIT behavior of annealed samples, which can be explained by a thermodynamic equilibrium state of the hydrogen movement.48

Figure 3. Annealing treatments of VO1.75(OH)0.25 in N2 gas flow. DSC curves of the VO1.75(OH)0.25 samples annealed (a) at different temperatures holding for 20 min in N2 and (b) at 325 °C for different times in N2. Plots of hysteresis width, phase transition temperature (Tc) versus (c) annealing temperature and (d) annealing time. (Note: phase transition temperature in DSC traces is defined at the intersection of the tangent to the maximum slope position of the peak and the extrapolated baseline, while the hysteresis width here is set as ∆Tc =Tc(Heating)- Tc(Cooling). In addition,


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the zero hysteresis width here doesn’t mean the complete elimination of hysteresis for the graded-doping VO2).

The heavily H-doped VO2 nanowires (sample B) are also applied annealing treatments in N2 gas flow to achieve rational phase transition modification and a deep understanding on the thermal movement of hydrogen/vacancies in the as-obtained nanocrystals, as demonstrated in Figure 4a and Figure 4b. Interestingly, the separate peaks in the original DSC traces tend to coalesce to form single peaks with increasing annealing temperature. With a further increase in annealing temperature or annealing time, the promotion of both Tc-h and Tc-c can be achieved. We believe that at a relatively low annealing temperature (< 260 °C), hydrogen cannot capture sufficient energy for the escape from hydric VO2 crystal lattice. Therefore, diffusion should be prominent to the thermal movement of hydrogen in this situation. According to the well-known diffusion mechanism, the redistribution of hydrogen can be driven by concentration gradient. However, the absolutely uniform distribution of hydrogen is not available in real situation, and a relatively ordered hydrogen concentration gradient can be finally obtained by the low-temperature annealing treatment of the as-prepared VO2 samples, which is revealed by the broad and continuous DSC peaks in Figure 4a and Figure 4b. At a relatively high temperature (> 260 °C), enough energy can be provided to the elimination of hydrogen, resulting in the overall promotion of the Tc of sample B. As to the thermal movement of vacancies, both the


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ejectment and diffusion (aggregation)49 may start at a relatively low annealing temperature, thus the effective influence of vacancies on the VO2 crystal lattice can be greatly

weakened

by increasing annealing temperature.

Consequently,

the

redistribution and elimination of hydrogen/vacancies via annealing are believed to assist the rational MIT modulation of as-prepared H-doped VO2 nanowires.

Figure 4. Annealing treatments of heavily H-doped VO2 samples. DSC curves of the sample B annealed under different conditions during (a) cooling cycles and (b) heating cycles, respectively. The blue arrows in (a) indicate the combination of two separate DSC peaks and the red arrows show the shift direction of the combined DSC exothermic peak. (c) DSC curves of the sample B experiencing the same heating


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treatment at 350 °C for 20min in air and in N2, respectively. (d) Optical images of a single heavily H-doped VO2 (M) nanowire (sample B) before and after the annealing in N2 upon heating, where the white parts are M phase and the green parts are R phase.

However, the simple annealing protected by inert gas is still limited. In the discussion above, the maximum Tc of ~ 50 °C can be obtained, which is far away from 68 °C, indicates the existence of residual hydrogen in the VO2 lattice, which can be revealed by the existence of R phase all along in the XRD patterns (Figure S5). Compared to the simple physical heating, oxidation is believed to be more efficient for the hydrogen elimination. As evidence, Figure 4c gives the DSC curves of the sample B that were equally annealed in air and in N2, respectively. Compared to the sample annealed in N2, the air-annealed VO2 sample demonstrates a higher Tc of 61 °C, larger specific heat and much sharper DSC peaks, which indicate that the elimination of hydrogen is more readily achievable by annealing in the presence of oxygen. Nevertheless, neither a higher annealing temperature nor a longer annealing time can further influence the MIT behavior of air-annealed VO2. It is found that a newly-formed compact V2O5 layer on the surface of nanowires (Figure S6) shields the nanowire from oxygen and thus impedes the further hydrogen elimination reaction.

From above results, the engineering on phase transition of as-fabricated products can be achieved by the simple annealing process with tunable Tc-h and Tc-c as well as


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variable hysteresis width. The specific MIT behavior of single annealed nanowires is revealed by optical observation. As shown in Figure 4d, compared to the complex multi-domains appearing in original hydric VO2 nanowires, an annealed nanowire from sample B demonstrates a unique single-domain MIT behavior, which indicates the spatial redistribution and elimination of hydrogen/vacancies driven by the annealing treatment. Interestingly, the M/R domain wall can monodirectionally move along the nanowire with monotonic increasing/decreasing ambient temperature. This phenomenon suggests an axially gradient H-doping along the nanowire, which provides a much wider MIT responding temperature window compared to un-doped or uniformly-doped VO2(M) nanowires.30 Furthermore, we have checked tens of annealed nanowires from sample A-C. It is found that all of the nanowires exhibited a similar “gradient” MIT behavior only with difference on their responding temperature windows, which are dependent on the corresponding initial doping and annealing conditions.

Considering the distinct optical contrast between the M and R phase domains, the annealed nanowires can be promisingly utilized as sensitive optically-readable thermometers on the measurement of nanoscale objects, which can indicate the ambient temperature with the position of domain wall just like mercury thermometers. As shown in Figure 5a, two thermometers are demonstrated: thermometer I (T1) was produced by the nanowire of sample B that was annealed in N2 at 350 °C for 20min, while thermometer II (T2) was produced by the nanowire of sample B that was annealed in air at 260 °C for 20min. It is found that there is an almost linear


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relationship between domain wall position (P, LR/LTotal) and surrounding temperature (T) in a single heating/cooling cycle. However, the existence of hysteresis results in a poor reversibility of as-built thermometers for thermal sensing. As discussed in the reported work,22,38 before the nanowires turn to full-R phase or full-M phase, the heating P-T curves can overlap with the cooling P-T curves (Figure 5b). Therefore, the hysteresis can be almost eliminated permitting reversible and linear P-T curves in certain temperature windows (reversible and linear temperature window, RLTW) and the reversible movement of domain wall has been verified by supplementary video1. Considering practical application, it is necessary to experimentally determine the RLTW as the operating range for each VO2 thermometer beforehand. Notably, VO2 thermometers have two RLTWs that are separately activated from full M state and full R state, corresponding to linear sections of the red curve and blue curve in Figure 5a, respectively. For application convenience, here we only select the RLTW at heating cycles as the default operating temperature range in the following. The RLTWs for the two chosen thermometers are in the range of 44-52 °C (T1) and 57-61 °C (T2). The reversible and linear movement of domain walls with fluctuating ambient temperature along the two thermometers has been confirmed by several rounds of heating and cooling cycles, respectively (Figure S7 and Figure S8). In addition, the diverse detection ranges of VO2 nanowire-based thermometers can be simply realized by modifying annealing conditions, which can satisfy the measurement requirement in different application environments.


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Figure 5. The performance of graded H-doping VO2 nanowire-based thermometers. (a) Plots of domain wall position along the graded-doping VO2 (M) nanowires versus surrounding temperature and the corresponding in situ optical images of an annealed thermometer (length=~45 µm) with increasing temperature, where the bright yellow domain is M phase and the dark grey part is R phase. (b) Reversible P-T curves of T1 and T2 during the incomplete phase transition, where each data point is calculated by averaging five rounds of heating-cooling cycles (SI, section S8) and the error bars are the corresponding standard deviations. (c) The temperature resolution versus detection temperature range and (d) the maximum relative sensitivity (Sm) versus the corresponding measurement temperature of graded H-doping VO2 nanowire (NW)-based thermometers in this work, compared to other


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types

of

micro/nano-scale

thermometers,

including

luminescence-based

thermometers,1-4 thermal-expansion-based thermometers,5,6 micro-thermocouples7-9 and micromechanical resonators.10

Relative sensitivity and temperature resolution are both crucial parameters for the precise thermal sensing at micro/nanoscale. According to the typical definition, the relative sensitivity (S=dQ/(QdT), where Q is the parameter responding to the temperature change and here is the length of R phase) of as-obtained nanowire-based thermometers can be obtained from the slope of the reversible P-T curves shown in Figure 5b, which are 6.15 %/K for T1 and 14.73 %/K for T2, respectively. Given the ultimate spatial resolution of optical microscope (~200 nm), it is discovered that T1 can detect a minimum temperature change of ~0.073 K and T2 can even achieve a temperature resolution of ~0.028 K (the calculation is described in SI, section S9). Even more excitingly, combined with advanced microscopic techniques, the VO2 nanowire-based thermometers in this work can provide ultrahigh temperature resolutions, such as ~10-4 K by SEM (spatial resolution~1 nm) and ~10-5 K by TEM (spatial resolution~0.1 nm). Additionally, other H-doping VO2 nanowire-based thermometers have been examined (Figure S9), which indicates the excellent repeatability of certain annealing processes to fabricate thermometers. Compared to the existing micro/nano-scale thermometry (Figure 5c and Figure 5d), the as-obtained thermometers demonstrate excellent performance in both temperature


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resolution and relative sensitivity. What’s more, the direct temperature measurement via optical microscope/electron microscope is much simpler to operate, meanwhile avoiding the complicated signal capture and analysis process. Additionally, the operation temperature of these thermometers is experimentally confirmed ranging from -20 to 61 °C via DSC (Figure S10), which can cover the commonly used physiological range (20 to 60 °C).50 Thus, we believe that the as-prepared VO2 nanowire-based thermometers with ultrahigh temperature resolution and sensitivity can enable extensive thermal biology studies in cell level.

Conclusions In summary, we have achieved the hydrothermal synthesis of intrinsic H-doping VO2 (M) nanowires with different sizes by tuning reaction conditions. During the hydrothermal reactions, the hydrogen-doping concentration of as-prepared VO2 nanowires can be adjusted by modifying the filling ratio and reductive agent concentration. Annealing treatments can be applied to help the redistribution and elimination of the dopants/vacancies in the as-grown hydric VO2 nanowires. As a result, the promotion of Tc and the reduction of hysteresis width can be simultaneously realized by properly increasing annealing temperature/time. It is also discovered that the annealed VO2 nanowires demonstrate clean and linear gradient MIT behavior, where only single domain-wall can be seen moving along the nanowire with ambient temperature. The unique gradient MIT behavior of the annealed can


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trigger wide studies of domain physics of the phase transition, as well as integrated device applications such as thermal sensing, actuators, memories, and optical switches. Upon that, sensitive optically-readable thermometers are demonstrated based on the annealed VO2 nanowires with ultra-high relative sensitivity (~17.4 %/K) and temperature resolution (~0.026 K) via optical microscope. Combined with TEM, an extremely high resolution of ~10-5 K can be even realized.

Note The authors declare no competing financial interest.

Acknowledgments The authors acknowledge the support by the Guangdong Natural Science Funds for Distinguished Young Scholars (Grant 2015A030306044), the Guangdong-Hong Kong joint innovation project (Grant No. 2016A050503012), the National Natural Science Foundation of China (Grants 51406075 and 51402147), the National Key Research and Development Project from the Ministry of Science and Technology (Grants 2016YFA0202400 and 2016YFA0202404), Training Program for Outstanding Young Teachers at Higher Education Institutions of Guangdong Province (Grant YQ2015151), Foundation of Shenzhen Science and Technology Innovation Committee (Grant JCYJ20150331101823695) and the Peacock Team Project from Shenzhen

Science

and

Technology

Innovation

Committee

(Grant

KQTD2015-033110182370). Starting grants from Southern University of Science and


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Technology are also acknowledged.

Supporting Information Available: Movie of nanowire-based thermometer upon several heating-cooling cycles; additional experimental data and characterization of as-prepared nanowires; theoretical analysis of the growth mechanism of VO2 nanowires by hydrothermal approaches; calculation to the relative sensitivity and temperature resolution of as-built thermometers. This information is available free of charge via the Internet at http://pubs.acs.org.

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