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Protection of boron nitride nanosheets by atomic layer deposition toward thermal energy management applications
Author’s Accepted Manuscript Protection of Boron Nitride Nanosheets by Atomic Layer Deposition toward Thermal Energy Management Applications Wei Luo, Lihui Zhou, Zhi Yang, Jiaqi Dai, Emily Hitz, Yudi Kuang, Xiaogang Han, Bao Yang, Liangbing Hu www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(17)30473-1 http://dx.doi.org/10.1016/j.nanoen.2017.08.004 NANOEN2118
To appear in: Nano Energy Received date: 10 May 2017 Revised date: 27 July 2017 Accepted date: 2 August 2017 Cite this article as: Wei Luo, Lihui Zhou, Zhi Yang, Jiaqi Dai, Emily Hitz, Yudi Kuang, Xiaogang Han, Bao Yang and Liangbing Hu, Protection of Boron Nitride Nanosheets by Atomic Layer Deposition toward Thermal Energy Management Applications, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.08.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Protection of Boron Nitride Nanosheets by Atomic Layer Deposition toward Thermal Energy Management Applications Wei Luo,1,2,‡ Lihui Zhou,1,‡ Zhi Yang,2,‡ Jiaqi Dai,1 Emily Hitz,1 Yudi Kuang,1 Xiaogang Han,1 Bao Yang,2 Liangbing Hu1,* 1
Department of Materials Science and Engineering, 2Department of Mechanical Engineering, University of Maryland, College Park, Maryland 20742, United States
Keywords: Thermal Energy Management, Boron Nitride Nanosheets, Atomic Layer Deposition, Selective Deposition, Thermally Conductive
‡These authors contributed equally. *Email: [email protected] Abstract Hexagonal boron nitride (h-BN), a material with long-known excellent thermal and chemical stability, has emerged as one of the most attractive two-dimensional materials for thermal energy management applications. However, recent discoveries revealed that oxidation of h-BN occurs at relatively low temperature due to the existence of defects on h-BN flakes. Herein, we propose a route to protect h-BN from oxidation at temperatures up to 1000 C by a thin layer of Al2O3 via atomic layer deposition (ALD). The ALDAl2O3 layer terminates the defects of BN and maintains BN intact at 1000 C under ambient atmosphere. In addition, a thermally conductive and electrically insulating glass has been demonstrated through a facile solution-processed BN nanosheets spray coating and subsequent Al2O3 deposition by ALD. The ALD-Al2O3 layer effectively glued the BN nanosheets and increased the adhesion between BN layer and glass, which enabled a high thermal conductivity of 4.55 W/m∙K while the bare glass shows a low thermal conductivity of 0.94 W/m∙K. Furthermore, with the BN and ALD-Al2O3 coating, more 1
uniform temperature distribution was obtained on the glass. We believe the coating approach by using BN and ALD technology is potentially competitive for scaling up in thermal energy management or high temperature applications.
Introduction As the analogue of graphite, hexagonal boron nitride (h-BN) has a layered structure with boron (B) and nitrogen (N) atoms occupying alternatingly in the planar hexagonal lattice.[1-3] Over the past several decades, h-BN has attracted great attention as a complementary to graphite.[4-8] Different from graphite, BN is a wide bandgap material (5.97 eV) due to the different on-site energies between B and N atoms, which result in the absence of free movement of electrons and a low electrical conductivity.[9, 10] Interestingly, BN exhibits a high thermal conductivity, where phonons are the dominant heat carriers. In addition, the great thermal/chemical stability of BN results in its widespread application in cements, lubricants and high temperature environments.[11-13] Recently, with the discovery of the extraordinary physical properties of graphene, fewlayer BN nanosheets (BNNSs) are expected to possess new potential and have attracted intense experimental and theoretical interest in heterostructure devices with other 2D materials including graphene, WSe2 and MoS2.[14-18] However, recent studies revealed that BNNSs start to oxidize at 700 ºC under ambient atmosphere.[19, 20] Since the thermal stability of BNNSs is critical for their high temperature applications, it is highly desirable to develop effective route to enhance the stability of BNNSs at elevated temperature.
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Atomic layer deposition (ALD) is an emerging thin film deposition technique for a range of applications in the semiconductor industry and the energy storage and conversion field.[21-26] Recently, ALD has also been developed as a functional tool to modify the surface properties for a variety of materials.[27-32] Zhao and co-workers demonstrated that coating planar Na metal with 25 cycles of ALD-Al2O3 layer can passivate Na metal in the electrolyte and greatly enhance its performance.[33] Sun et al. discovered that carbon nanotubes (CNTs) could be effectively protected from oxidation by a nanometerthick ALD-Al2O3 coating.[34] Inspired by these studies, herein we investigate the stability of BN at high temperature and protection strategies by ALD coating. We discovered that pits and holes appear on defects of pristine BN flakes at 1000 ºC and extended annealing duration leads to much larger and deeper holes on the surface and etching on the edges. After depositing a thin Al2O3 layer (50 nm) by ALD, BN remains intact from oxidation at 1000 ºC, proving the protective role of ALD-Al2O3 in terminating the defects of BN (Figure 1). More interestingly, a thin layer of BNNSs and ALD-Al2O3 can significantly improve the thermal conductivity and heat distribution ability of glass, providing a great potential for broadening the applications of glass.
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Figure 1. Schematic of ALD-Al2O3 coating on defects of BN can effectively maintains BN intact from oxidation while a large number of pores are generated on pristine BN flakes. Materials and Methods Materials Pristine BN flake was provided by Momentive Inc. and used as received without further purification. In a typical annealing process, BN flakes (200 mg) was placed in a tube furnace and heated to a temperature between 8001000 ºC (heating rate: 2 ºC/min) for a given period of time under air flow. To prepare BN-coated glass, a BN nanosheetsisopropyl alcohol (IPA) ink was prepared using a sonication-assisted exfoliation method.[9] The BN ink was then loaded into a spray gun and spray-coated onto a glass substrate using nitrogen as carrier gas. With a heating source under the glass substrate, the BN coating could be finished in few minutes. To conduct the Al2O3 coating, pristine BN flake or BN-coated glass was transferred to a Beneq TFS 500 ALD tool with a 4
growth rate of 1.2 Å/cycle. Precursors used for the coating were trimethylaluminum (TMA) and H2O, and the ALD process temperature was 150 C. The thickness of the ALD Al2O3 coating layer was measured by ellipsometric measurements using a blank Si wafer as the standard substrate. The typical thickness of Al2O3 layer is 50 nm. Characterization The morphologies were measured by a field-emission scanning electron microscope (SEM, Hitachi SU-70). Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images were collected using a JEOL TEM (JEM 2100). X-ray photoelectron spectroscopy (XPS) analysis were conducted using a Kratos Axis 165 Xray photoelectron spectrometer operating in hybrid mode using monochromatic Al KR X-rays (1486.7 eV). The in-plane thermal conductivity was measured using a steady state method as reported in our previous study.[9] To compare the heat distribution ability of glass and ALD-Al2O3-BN coated glass, a Coherent Highlight FAP 100 laser system with a central wavelength of 810nm was used to generate the heat spot and an infrared (IR) camera (FLIR Merlin MID) was used to map the temperature distribution.
Results and Discussion The stability of BN was investigated in detail by characterizing the morphology of pristine BN flakes before and after annealing at various temperatures and durations. The obtained samples are referred to as BN-T-X, where T is the annealing temperature and X is the annealing duration, as listed in Table 1. Figure 2a shows a low-magnification SEM image of the pristine BN flakes, which exhibits typical platelet morphology with size ranging from several microns to about 100 μm. The high-magnification SEM image
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confirms that BN flakes are formed by stacked BN sheets each having a smooth surface (Figure 2b). After annealing at 800 ºC and 900 ºC for 1 h, no clear morphology change can be observed, which suggests bulk BN flake is stable up to 900 ºC (Figure S1 and Figure S2). When the annealing temperature is increased to 1000 ºC, the overall morphology remains the same (Figure 2c) while a large number of holes are found on the surface of BN-1000-1h (Figure 2d), indicating that pristine BN flake is unstable in air at high temperature. These holes have sizes ranging from 100 nm to ~1 μm and some holes exhibit a well-defined hexagonal shape with side length of ~100 nm. We then study the duration dependence of the stability, where BN flake is first heated to 1000 ºC without holding at this temperature to give BN-1000-0h. Some tiny pits with irregular shape can be seen, showing that oxidation starts at 1000 ºC (Figure S3). When the duration was extended to 0.5 h, large quantities of well-defined hexagonal holes with side lengths ranging from 30 to 50 nm are observed (Figure 2e and 2f). Compared with BN-1000-1h, these holes on BN-1000-0.5h are smaller and more monodisperse. TEM observations confirm that many individual hexagonal holes are dispersed on BN-1000-0.5h (Figure 2g and 2h). A typical HRTEM image of BN-1000-0.5h shows the (100) plane of cubic BN phase, which indicates that intact parts are still pure BN (Figure 2i).
Table 1. Experimental conditions and chemical compositions of BN samples. Sample Name Pristine BN flake BN-800-1h BN-900-1h BN-1000-0h BN-1000-0.5h
Temperature (ºC)
Duration (h)
800 900 1000 1000
1 1 0 0.5
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XPS B (at%) 48.8% 48.2% 47.6% N/A 47.1%
XPS N (at%) 48.6% 48.6% 45.0% N/A 40.1%
XPS O (at%) 2.6% 3.2% 7.4% N/A 12.8%
BN-1000-1h BN-1000-3h BN-1000-6h
1000 1000 1000
1 3 6
45.6% 44.5% 42.6%
32.6% 30.6% 13.2%
21.8% 24.9% 44.2%
Figure 2. Morphology observation of pristine BN flakes before and after thermal treatment in air. SEM images of (a, b) pristine BN flakes, (c, d) BN-1000-1h and (e, f) BN-1000-0.5h, showing the formation of holes on BN flakes by annealing BN flakes at 1000 ºC. (g, h) Low and high-magnification TEM images of BN-1000-0.5h, suggesting well-defined, monodispersed hexagonal holes on BN flakes. (i) A HRTEM image of BN1000-0.5h, confirming the (100) plane of cubic BN phase on intact parts of BN.
The above observation indicates that well-defined, monodispersed holes can be obtained on BN. Since porous materials can outperform their bulk counterparts in a range of 7
applications, our findings may shed light on studies of producing porous BN and extend to prepare other porous 2D materials.[35-39] When further extending the annealing duration to 6 hours, the surface morphology changes drastically. As marked in Figure 3a and 3b, edges of BN were seriously corroded and holes on the surface became much larger and deeper, suggesting violent oxidation occurred. To further examine the surface chemical compositions, XPS spectra were further collected. As shown in Figure S4, pristine BN flake shows two main peaks for boron (B) and nitrogen (N), giving an atomic ratio of ~1.0 (Table 1). Minuscule oxygen (O) and carbon (C) peaks were also detected, which should be due to few impurities in the raw material. After annealing at 800 ºC for 1h, the chemical composition is almost the same, indicating that oxidation does not occur yet. For BN-900-1h, the composition of O is increased to 7.39%. Combining with the SEM results, the slight increase of O signal may result from mild oxidation on the defects/edges of BN at 900 ºC, which would not lead to the formation of holes on the surface. As discussed above, the morphology change occurred at 1000 ºC and violently intensified with an increase in annealing duration. The concentration of O also follows this trend, gradually increasing from 12.83% for BN-1000-0.5h, 21.81% for BN-1000-1h, 24.95% for BN-1000-3h, to 44.25% for BN-1000-6h (Table 1). On the other hand, the N peak decreases stepwise with the increase of O, suggesting that N is removed from BN surface during the oxidation of BN at 1000 ºC in air. From the morphology observation and XPS results, it is clear that defects and edges of BN are oxidized in air at 1000 ºC.[20] To repair the defects and edges, in this work, we coated pristine BN flakes by a thin layer of Al2O3 via ALD (50 nm). Interestingly, the morphology of BN flake was maintained well after ALD coating due to its thin thickness
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(50 nm, Figure S5). To test the protection effect of the ALD-Al2O3 layer against oxidization, we annealed ALD-Al2O3 coated BN at 1000 ºC in air for 6h. Notably, SEM images reveal that there is no pores or etching occurring on BN flakes when the Al2O3 was deposited by ALD (Figure 3c and 3d). This confirms the significantly protective role of ALD-Al2O3 coating layer for BN flakes under high temperature in air. According to previous reports,[20, 34] oxidation of BN or other materials starts at their defects and edges, the selective deposition of Al2O3 by ALD is promising to improve the stability of various materials.
Figure 3. (a, b) SEM images of BN-1000-6h, showing violent etching occurs on edges of BN and larger/deeper pores form on pristine BN flakes. (c, d) SEM images of ALDAl2O3 coated BN flakes after annealing at 1000 ºC for 6 hours. No clear oxidation occurred on ALD-Al2O3 coated BN, indicating the protective role of the ALD-Al2O3 coating layer.
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Both BN and Al2O3 have a high thermal conductivity and low electrical conductivity, which motivate us to apply BN and ALD-Al2O3 as coating materials for improving the thermal properties of other materials.[40] In modern life, glass is one of the most widely used substrates and its poor thermal-related properties significantly limits its applications. As a proof-of-concept study, here we demonstrate the application of BN and ALD-Al2O3 on glass substrate. BN nanosheets with a relatively uniform thickness and regular shaped flakes (300-600 nm) was first prepared by a sonication-exfoliation method based on our pervious report (Figure S6)[9] and spray-coated onto glass substrate with a thickness of ~5 μm (Figure 4a). Transparent glass turns white after spray coating (Figure 4b). SEM images confirm that the BN layer comprises a large number of nanosheets (Figure 4d and 4e). After coating a thin layer of Al2O3 via ALD (50 nm), the color of glass turns slightly grey and the bulk morphology remains unchanged (Figure 4c and 4g). When zoomed in, the BN layer becomes more continuous after ALD-Al2O3 coating (Figure 4h), where the ALD-Al2O3 coating acts as a glue to improve the connection between the BN nanosheets. In addition, the ALD-Al2O3 layer also effectively bonded the BN layer with glass together (Figure S7). As confirmed in Figure 4f and 4i, the BN layer is easily peeled off from glass by scotch tape, suggesting a poor adhesion. On the other hand, after depositing an additional ALD-Al2O3 layer, scotch tape cannot peel off the BN layer (Figure 4i).
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Figure 4. (a) Schematic of the spray coating technique used to apply a BN layer to glass substrate. (b, c) Digital photos and (d, e, g, h) SEM images of BN coated glass (b, d, e) and ALD-Al2O3-BN coated glass (c, g, h). Scotch tape testing for (f) BN coated glass and (i) ALD-Al2O3-BN coated glass.
Enabled by the coating, the thermal properties of the glass were expected to improve. The thermal conductivity is measured using a steady-state method outlined in our previous studies.[9] The pristine glass shows a low thermal conductivity of 0.94 W/m∙K. After coating with BN, the thermal conductivity increased to 3.85 W/m∙K, which is about four times higher than pristine glass. An additional ALD-Al2O3 layer further increased the thermal conductivity to 4.55 W/m∙K (Figure 5a). Such an increase is due to the more 11
continuous structure created by the ALD-Al2O3 coating. Note that the thermal conductivities of BN coated glass and ALD-Al2O3-BN coated glass were calculated based on the whole thickness including glass substrate. To demonstrate the effects of the enhanced thermal conductivity, an IR camera is used to test the temperature distribution on the glass substrate before and after coating by a focused laser beam (Figure 5b, detailed measurement is described in Supporting Information). As shown in Figure 5c and 5d, the temperature spike is about 38.4 °C at the hotspot in the pristine glass. In comparison, the temperature spike is reduced to 36.1 °C due to the enhanced heat spreading with the ALD-Al2O3-BNs coating. The ALD-Al2O3-BN coated glass can create a more uniform thermal distribution. Although the transparency of glass plate was changed due to the coating, the thermal property measurement demonstrated the concept that the unique coating technique of BN and ALD-A2O3 can be an effective method to improve the thermal conductivity of target materials. We believe that this unique coating technology can be further extended to other materials such as ceramics.
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Figure 5. (a) Thermal conductivity of pristine glass and ALD-Al2O3-BN coated glass. (b) Schematic illustration of mapping the temperature distribution when a hotspot is created on the glasses by a focused laser beam; (c, d) temperature distribution images on pristine glass and ALD-Al2O3-BN coated glass. A lower temperature spike can be observed on ALD-Al2O3-BN coated glass, demonstrating a better ability to distribute heat.
Conclusion In summary, we demonstrate that pristine BN flake is not stable at 1000 C in air due to the oxidation of BN on defective areas and edges. Well-defined, hexagonal, monodispersed holes can be obtained on BN by controlling annealing conditions, which can shed light on studies of producing porous BN and extend to prepare other porous 2D materials. A strategy to protect BN from oxidation at high temperature by ALD coating 13
was presented. Furthermore, we demonstrated a thermally conductive, electrically insulating coating based on BN and ALD. We find that the ALD-Al2O3 coating can effectively glue the BN layer and improve the adhesion between BN and glass. In addition, the ALD-Al2O3-BN coated glass shows a much-enhanced thermal conductivity and thermal distribution compared to pristine glass, indicating a great potential for many applications. We believe our strategy for producing a thermally conductive material coating can be regarded as a general coating method for various substrates to enhance their corresponding thermal properties. The conformal ALD oxide coating developed in this study can also be applied for other materials to enhance their corresponding thermal performance.
Acknowledgement This work was funded by the Office of Naval Research (ONR) under grant N000141410721 and the ONR Young Investigator Program (YIP, L. Hu). The authors acknowledge the support of the Maryland Nanocenter and its Fablab, Nisplab, and surface analysis center. The authors also acknowledge Momentive Inc. for providing pristine BN flakes.
Supporting Information. Supplementary data associated with this article can be found in the online version.
Notes The authors declare no competing financial interest.
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Highlights
Pits and holes appear on pristine BN flakes at high temperature;
Well-defined, monodispersed holes can be obtained on BN;
A thin Al2O3 layer (50 nm) by Atomic Layer Deposition (ALD) plays a protective role for improving the thermal stability of BN flakes;
A thermally conductive, electrically insulating glass can be achieved based on BN and ALD.
Graphical Abstract
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PII: DOI: Reference:
S2211-2855(17)30473-1 http://dx.doi.org/10.1016/j.nanoen.2017.08.004 NANOEN2118
To appear in: Nano Energy Received date: 10 May 2017 Revised date: 27 July 2017 Accepted date: 2 August 2017 Cite this article as: Wei Luo, Lihui Zhou, Zhi Yang, Jiaqi Dai, Emily Hitz, Yudi Kuang, Xiaogang Han, Bao Yang and Liangbing Hu, Protection of Boron Nitride Nanosheets by Atomic Layer Deposition toward Thermal Energy Management Applications, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.08.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Protection of Boron Nitride Nanosheets by Atomic Layer Deposition toward Thermal Energy Management Applications Wei Luo,1,2,‡ Lihui Zhou,1,‡ Zhi Yang,2,‡ Jiaqi Dai,1 Emily Hitz,1 Yudi Kuang,1 Xiaogang Han,1 Bao Yang,2 Liangbing Hu1,* 1
Department of Materials Science and Engineering, 2Department of Mechanical Engineering, University of Maryland, College Park, Maryland 20742, United States
Keywords: Thermal Energy Management, Boron Nitride Nanosheets, Atomic Layer Deposition, Selective Deposition, Thermally Conductive
‡These authors contributed equally. *Email: [email protected] Abstract Hexagonal boron nitride (h-BN), a material with long-known excellent thermal and chemical stability, has emerged as one of the most attractive two-dimensional materials for thermal energy management applications. However, recent discoveries revealed that oxidation of h-BN occurs at relatively low temperature due to the existence of defects on h-BN flakes. Herein, we propose a route to protect h-BN from oxidation at temperatures up to 1000 C by a thin layer of Al2O3 via atomic layer deposition (ALD). The ALDAl2O3 layer terminates the defects of BN and maintains BN intact at 1000 C under ambient atmosphere. In addition, a thermally conductive and electrically insulating glass has been demonstrated through a facile solution-processed BN nanosheets spray coating and subsequent Al2O3 deposition by ALD. The ALD-Al2O3 layer effectively glued the BN nanosheets and increased the adhesion between BN layer and glass, which enabled a high thermal conductivity of 4.55 W/m∙K while the bare glass shows a low thermal conductivity of 0.94 W/m∙K. Furthermore, with the BN and ALD-Al2O3 coating, more 1
uniform temperature distribution was obtained on the glass. We believe the coating approach by using BN and ALD technology is potentially competitive for scaling up in thermal energy management or high temperature applications.
Introduction As the analogue of graphite, hexagonal boron nitride (h-BN) has a layered structure with boron (B) and nitrogen (N) atoms occupying alternatingly in the planar hexagonal lattice.[1-3] Over the past several decades, h-BN has attracted great attention as a complementary to graphite.[4-8] Different from graphite, BN is a wide bandgap material (5.97 eV) due to the different on-site energies between B and N atoms, which result in the absence of free movement of electrons and a low electrical conductivity.[9, 10] Interestingly, BN exhibits a high thermal conductivity, where phonons are the dominant heat carriers. In addition, the great thermal/chemical stability of BN results in its widespread application in cements, lubricants and high temperature environments.[11-13] Recently, with the discovery of the extraordinary physical properties of graphene, fewlayer BN nanosheets (BNNSs) are expected to possess new potential and have attracted intense experimental and theoretical interest in heterostructure devices with other 2D materials including graphene, WSe2 and MoS2.[14-18] However, recent studies revealed that BNNSs start to oxidize at 700 ºC under ambient atmosphere.[19, 20] Since the thermal stability of BNNSs is critical for their high temperature applications, it is highly desirable to develop effective route to enhance the stability of BNNSs at elevated temperature.
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Atomic layer deposition (ALD) is an emerging thin film deposition technique for a range of applications in the semiconductor industry and the energy storage and conversion field.[21-26] Recently, ALD has also been developed as a functional tool to modify the surface properties for a variety of materials.[27-32] Zhao and co-workers demonstrated that coating planar Na metal with 25 cycles of ALD-Al2O3 layer can passivate Na metal in the electrolyte and greatly enhance its performance.[33] Sun et al. discovered that carbon nanotubes (CNTs) could be effectively protected from oxidation by a nanometerthick ALD-Al2O3 coating.[34] Inspired by these studies, herein we investigate the stability of BN at high temperature and protection strategies by ALD coating. We discovered that pits and holes appear on defects of pristine BN flakes at 1000 ºC and extended annealing duration leads to much larger and deeper holes on the surface and etching on the edges. After depositing a thin Al2O3 layer (50 nm) by ALD, BN remains intact from oxidation at 1000 ºC, proving the protective role of ALD-Al2O3 in terminating the defects of BN (Figure 1). More interestingly, a thin layer of BNNSs and ALD-Al2O3 can significantly improve the thermal conductivity and heat distribution ability of glass, providing a great potential for broadening the applications of glass.
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Figure 1. Schematic of ALD-Al2O3 coating on defects of BN can effectively maintains BN intact from oxidation while a large number of pores are generated on pristine BN flakes. Materials and Methods Materials Pristine BN flake was provided by Momentive Inc. and used as received without further purification. In a typical annealing process, BN flakes (200 mg) was placed in a tube furnace and heated to a temperature between 8001000 ºC (heating rate: 2 ºC/min) for a given period of time under air flow. To prepare BN-coated glass, a BN nanosheetsisopropyl alcohol (IPA) ink was prepared using a sonication-assisted exfoliation method.[9] The BN ink was then loaded into a spray gun and spray-coated onto a glass substrate using nitrogen as carrier gas. With a heating source under the glass substrate, the BN coating could be finished in few minutes. To conduct the Al2O3 coating, pristine BN flake or BN-coated glass was transferred to a Beneq TFS 500 ALD tool with a 4
growth rate of 1.2 Å/cycle. Precursors used for the coating were trimethylaluminum (TMA) and H2O, and the ALD process temperature was 150 C. The thickness of the ALD Al2O3 coating layer was measured by ellipsometric measurements using a blank Si wafer as the standard substrate. The typical thickness of Al2O3 layer is 50 nm. Characterization The morphologies were measured by a field-emission scanning electron microscope (SEM, Hitachi SU-70). Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images were collected using a JEOL TEM (JEM 2100). X-ray photoelectron spectroscopy (XPS) analysis were conducted using a Kratos Axis 165 Xray photoelectron spectrometer operating in hybrid mode using monochromatic Al KR X-rays (1486.7 eV). The in-plane thermal conductivity was measured using a steady state method as reported in our previous study.[9] To compare the heat distribution ability of glass and ALD-Al2O3-BN coated glass, a Coherent Highlight FAP 100 laser system with a central wavelength of 810nm was used to generate the heat spot and an infrared (IR) camera (FLIR Merlin MID) was used to map the temperature distribution.
Results and Discussion The stability of BN was investigated in detail by characterizing the morphology of pristine BN flakes before and after annealing at various temperatures and durations. The obtained samples are referred to as BN-T-X, where T is the annealing temperature and X is the annealing duration, as listed in Table 1. Figure 2a shows a low-magnification SEM image of the pristine BN flakes, which exhibits typical platelet morphology with size ranging from several microns to about 100 μm. The high-magnification SEM image
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confirms that BN flakes are formed by stacked BN sheets each having a smooth surface (Figure 2b). After annealing at 800 ºC and 900 ºC for 1 h, no clear morphology change can be observed, which suggests bulk BN flake is stable up to 900 ºC (Figure S1 and Figure S2). When the annealing temperature is increased to 1000 ºC, the overall morphology remains the same (Figure 2c) while a large number of holes are found on the surface of BN-1000-1h (Figure 2d), indicating that pristine BN flake is unstable in air at high temperature. These holes have sizes ranging from 100 nm to ~1 μm and some holes exhibit a well-defined hexagonal shape with side length of ~100 nm. We then study the duration dependence of the stability, where BN flake is first heated to 1000 ºC without holding at this temperature to give BN-1000-0h. Some tiny pits with irregular shape can be seen, showing that oxidation starts at 1000 ºC (Figure S3). When the duration was extended to 0.5 h, large quantities of well-defined hexagonal holes with side lengths ranging from 30 to 50 nm are observed (Figure 2e and 2f). Compared with BN-1000-1h, these holes on BN-1000-0.5h are smaller and more monodisperse. TEM observations confirm that many individual hexagonal holes are dispersed on BN-1000-0.5h (Figure 2g and 2h). A typical HRTEM image of BN-1000-0.5h shows the (100) plane of cubic BN phase, which indicates that intact parts are still pure BN (Figure 2i).
Table 1. Experimental conditions and chemical compositions of BN samples. Sample Name Pristine BN flake BN-800-1h BN-900-1h BN-1000-0h BN-1000-0.5h
Temperature (ºC)
Duration (h)
800 900 1000 1000
1 1 0 0.5
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XPS B (at%) 48.8% 48.2% 47.6% N/A 47.1%
XPS N (at%) 48.6% 48.6% 45.0% N/A 40.1%
XPS O (at%) 2.6% 3.2% 7.4% N/A 12.8%
BN-1000-1h BN-1000-3h BN-1000-6h
1000 1000 1000
1 3 6
45.6% 44.5% 42.6%
32.6% 30.6% 13.2%
21.8% 24.9% 44.2%
Figure 2. Morphology observation of pristine BN flakes before and after thermal treatment in air. SEM images of (a, b) pristine BN flakes, (c, d) BN-1000-1h and (e, f) BN-1000-0.5h, showing the formation of holes on BN flakes by annealing BN flakes at 1000 ºC. (g, h) Low and high-magnification TEM images of BN-1000-0.5h, suggesting well-defined, monodispersed hexagonal holes on BN flakes. (i) A HRTEM image of BN1000-0.5h, confirming the (100) plane of cubic BN phase on intact parts of BN.
The above observation indicates that well-defined, monodispersed holes can be obtained on BN. Since porous materials can outperform their bulk counterparts in a range of 7
applications, our findings may shed light on studies of producing porous BN and extend to prepare other porous 2D materials.[35-39] When further extending the annealing duration to 6 hours, the surface morphology changes drastically. As marked in Figure 3a and 3b, edges of BN were seriously corroded and holes on the surface became much larger and deeper, suggesting violent oxidation occurred. To further examine the surface chemical compositions, XPS spectra were further collected. As shown in Figure S4, pristine BN flake shows two main peaks for boron (B) and nitrogen (N), giving an atomic ratio of ~1.0 (Table 1). Minuscule oxygen (O) and carbon (C) peaks were also detected, which should be due to few impurities in the raw material. After annealing at 800 ºC for 1h, the chemical composition is almost the same, indicating that oxidation does not occur yet. For BN-900-1h, the composition of O is increased to 7.39%. Combining with the SEM results, the slight increase of O signal may result from mild oxidation on the defects/edges of BN at 900 ºC, which would not lead to the formation of holes on the surface. As discussed above, the morphology change occurred at 1000 ºC and violently intensified with an increase in annealing duration. The concentration of O also follows this trend, gradually increasing from 12.83% for BN-1000-0.5h, 21.81% for BN-1000-1h, 24.95% for BN-1000-3h, to 44.25% for BN-1000-6h (Table 1). On the other hand, the N peak decreases stepwise with the increase of O, suggesting that N is removed from BN surface during the oxidation of BN at 1000 ºC in air. From the morphology observation and XPS results, it is clear that defects and edges of BN are oxidized in air at 1000 ºC.[20] To repair the defects and edges, in this work, we coated pristine BN flakes by a thin layer of Al2O3 via ALD (50 nm). Interestingly, the morphology of BN flake was maintained well after ALD coating due to its thin thickness
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(50 nm, Figure S5). To test the protection effect of the ALD-Al2O3 layer against oxidization, we annealed ALD-Al2O3 coated BN at 1000 ºC in air for 6h. Notably, SEM images reveal that there is no pores or etching occurring on BN flakes when the Al2O3 was deposited by ALD (Figure 3c and 3d). This confirms the significantly protective role of ALD-Al2O3 coating layer for BN flakes under high temperature in air. According to previous reports,[20, 34] oxidation of BN or other materials starts at their defects and edges, the selective deposition of Al2O3 by ALD is promising to improve the stability of various materials.
Figure 3. (a, b) SEM images of BN-1000-6h, showing violent etching occurs on edges of BN and larger/deeper pores form on pristine BN flakes. (c, d) SEM images of ALDAl2O3 coated BN flakes after annealing at 1000 ºC for 6 hours. No clear oxidation occurred on ALD-Al2O3 coated BN, indicating the protective role of the ALD-Al2O3 coating layer.
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Both BN and Al2O3 have a high thermal conductivity and low electrical conductivity, which motivate us to apply BN and ALD-Al2O3 as coating materials for improving the thermal properties of other materials.[40] In modern life, glass is one of the most widely used substrates and its poor thermal-related properties significantly limits its applications. As a proof-of-concept study, here we demonstrate the application of BN and ALD-Al2O3 on glass substrate. BN nanosheets with a relatively uniform thickness and regular shaped flakes (300-600 nm) was first prepared by a sonication-exfoliation method based on our pervious report (Figure S6)[9] and spray-coated onto glass substrate with a thickness of ~5 μm (Figure 4a). Transparent glass turns white after spray coating (Figure 4b). SEM images confirm that the BN layer comprises a large number of nanosheets (Figure 4d and 4e). After coating a thin layer of Al2O3 via ALD (50 nm), the color of glass turns slightly grey and the bulk morphology remains unchanged (Figure 4c and 4g). When zoomed in, the BN layer becomes more continuous after ALD-Al2O3 coating (Figure 4h), where the ALD-Al2O3 coating acts as a glue to improve the connection between the BN nanosheets. In addition, the ALD-Al2O3 layer also effectively bonded the BN layer with glass together (Figure S7). As confirmed in Figure 4f and 4i, the BN layer is easily peeled off from glass by scotch tape, suggesting a poor adhesion. On the other hand, after depositing an additional ALD-Al2O3 layer, scotch tape cannot peel off the BN layer (Figure 4i).
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Figure 4. (a) Schematic of the spray coating technique used to apply a BN layer to glass substrate. (b, c) Digital photos and (d, e, g, h) SEM images of BN coated glass (b, d, e) and ALD-Al2O3-BN coated glass (c, g, h). Scotch tape testing for (f) BN coated glass and (i) ALD-Al2O3-BN coated glass.
Enabled by the coating, the thermal properties of the glass were expected to improve. The thermal conductivity is measured using a steady-state method outlined in our previous studies.[9] The pristine glass shows a low thermal conductivity of 0.94 W/m∙K. After coating with BN, the thermal conductivity increased to 3.85 W/m∙K, which is about four times higher than pristine glass. An additional ALD-Al2O3 layer further increased the thermal conductivity to 4.55 W/m∙K (Figure 5a). Such an increase is due to the more 11
continuous structure created by the ALD-Al2O3 coating. Note that the thermal conductivities of BN coated glass and ALD-Al2O3-BN coated glass were calculated based on the whole thickness including glass substrate. To demonstrate the effects of the enhanced thermal conductivity, an IR camera is used to test the temperature distribution on the glass substrate before and after coating by a focused laser beam (Figure 5b, detailed measurement is described in Supporting Information). As shown in Figure 5c and 5d, the temperature spike is about 38.4 °C at the hotspot in the pristine glass. In comparison, the temperature spike is reduced to 36.1 °C due to the enhanced heat spreading with the ALD-Al2O3-BNs coating. The ALD-Al2O3-BN coated glass can create a more uniform thermal distribution. Although the transparency of glass plate was changed due to the coating, the thermal property measurement demonstrated the concept that the unique coating technique of BN and ALD-A2O3 can be an effective method to improve the thermal conductivity of target materials. We believe that this unique coating technology can be further extended to other materials such as ceramics.
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Figure 5. (a) Thermal conductivity of pristine glass and ALD-Al2O3-BN coated glass. (b) Schematic illustration of mapping the temperature distribution when a hotspot is created on the glasses by a focused laser beam; (c, d) temperature distribution images on pristine glass and ALD-Al2O3-BN coated glass. A lower temperature spike can be observed on ALD-Al2O3-BN coated glass, demonstrating a better ability to distribute heat.
Conclusion In summary, we demonstrate that pristine BN flake is not stable at 1000 C in air due to the oxidation of BN on defective areas and edges. Well-defined, hexagonal, monodispersed holes can be obtained on BN by controlling annealing conditions, which can shed light on studies of producing porous BN and extend to prepare other porous 2D materials. A strategy to protect BN from oxidation at high temperature by ALD coating 13
was presented. Furthermore, we demonstrated a thermally conductive, electrically insulating coating based on BN and ALD. We find that the ALD-Al2O3 coating can effectively glue the BN layer and improve the adhesion between BN and glass. In addition, the ALD-Al2O3-BN coated glass shows a much-enhanced thermal conductivity and thermal distribution compared to pristine glass, indicating a great potential for many applications. We believe our strategy for producing a thermally conductive material coating can be regarded as a general coating method for various substrates to enhance their corresponding thermal properties. The conformal ALD oxide coating developed in this study can also be applied for other materials to enhance their corresponding thermal performance.
Acknowledgement This work was funded by the Office of Naval Research (ONR) under grant N000141410721 and the ONR Young Investigator Program (YIP, L. Hu). The authors acknowledge the support of the Maryland Nanocenter and its Fablab, Nisplab, and surface analysis center. The authors also acknowledge Momentive Inc. for providing pristine BN flakes.
Supporting Information. Supplementary data associated with this article can be found in the online version.
Notes The authors declare no competing financial interest.
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Highlights
Pits and holes appear on pristine BN flakes at high temperature;
Well-defined, monodispersed holes can be obtained on BN;
A thin Al2O3 layer (50 nm) by Atomic Layer Deposition (ALD) plays a protective role for improving the thermal stability of BN flakes;
A thermally conductive, electrically insulating glass can be achieved based on BN and ALD.
Graphical Abstract
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