One Step Nanoscale Patterning of Silver Nanowire-Nitride

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C: Physical Processes in Nanomaterials and Nanostructures

One Step Nanoscale Patterning of Silver Nanowire-Nitride Heterostructures using Substrate-Assisted Chemical Etching Christopher Elbadawi, Johannes E. Froech, Igor Aharonovich, Milos Toth, and Charlene J Lobo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10645 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

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The Journal of Physical Chemistry

One Step Nanoscale Patterning of Silver NanowireNitride Heterostructures Using Substrate-Assisted Chemical Etching Christopher Elbadawi, Johannes E. Fröch, Igor Aharonovich, Milos Toth, Charlene J. Lobo* AUTHOR ADDRESS School of Mathematical and Physical Sciences, University of Technology Sydney, Ultimo, NSW, 2007, Australia

ABSTRACT Nanoscale etching and patterning of noble metals such as copper, silver and gold is extremely difficult to achieve due to the low volatility of group 11 metal compounds. Here, we introduce a method of nanoscale chemical etching that utilizes reactions between H2O adsorbates and N radicals generated from electron beam-induced etching (EBIE) of a h-BN or AlN substrate to achieve efficient and highly localized chemical etching of Ag nanowires and the underlying substrate. The volatilization of noble metal nanowires by radical species generated during EBIE of the underlying substrate represents a new class of EBIE reactions, which we term ‘substrateassisted chemical etching’. INTRODUCTION Thin film heterostructures of metals and two-dimensional (2D) materials such as graphene and hexagonal boron nitride (hBN) are of high interest in flexible optoelectronic devices such as 1

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photodetectors1, single-photon sources2-3, resistive memories4, and light emitting diodes5. In many devices, metals such as Au and Ag are employed as plasmonic elements6-8 or transparent conducting electrodes5, while hBN is typically employed as a substrate, cavity or encapsulation material due to its high optical transmittance, excellent mechanical properties and high stability9. Fabrication of devices based on metal/hBN heterostructures generally requires multi-step processes in which the nitride and metal layers are sputtered and plasma etched in separate steps using different process gases10-12. While many metals can be etched and patterned using halogenated plasmas10, the low volatility of group 11 (Cu, Ag and Au) halides has made nanoscale etching and patterning of these metals extremely difficult to achieve. In this work, we introduce a method of patterning with nanoscale resolution using a novel single-step chemical process that utilizes N radicals originating from a nitride substrate in addition to a H2O precursor gas as reactants for focused electron beam induced etching13. EBIE has previously been employed to achieve 3D nanoprinting of plasmonic structures14-15, pattern formation16 and surface functionalization17 of diamond, fabrication and tuning of photonic crystal cavities9, 18, fabrication of 3D magnetic components19 and patterning of hBN20. Here, the highly localized formation of O, H and N radicals by H2O EBIE of nitride substrates using a focused electron beam is used to achieve nanoscale chemical etching of Ag nanowire-hBN and Ag nanowire-AlN heterostructures in a single step. This new etch method also has potential to be extended to other metal-nitride heterostructures (such as InGaN/GaN nanorod waveguides coupled to an Au film21) employed in plasmonic and photonic devices. EXPERIMENTAL

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Bulk hBN was grown by a high temperature, high pressure process22 then mechanically exfoliated onto a Si(111) substrate. AlN (0.5 mm thickness) was supplied by MTI corporation, and GaN substrates were grown using Metal-Organic Chemical Vapour Deposition (MOCVD)23. The substrates were ultrasonicated in acetone, IPA, and ethanol and then dried under flowing N2. Single crystal Ag nanowires were prepared with ascorbic acid (AA) and deionized water24. The Ag nanowire suspension was diluted in IPA, then dropcast onto the hBN, AlN or GaN substrate prior to EBIE experiments. EBIE experiments were performed using a variable pressure FEI Nova NanoSEM. Ag nanowires were dropcast onto various substrates (including hBN, GaN, AlN, and oxide-covered Si-see Table 1), loaded into the NanoSEM and pumped to high vacuum (3 x 10-4 Pa). The chamber was filled with a low pressure (8-16 Pa) of gas, and the Ag nanowires were located using a gas cascade magnetic field assisted detector. Unless otherwise stated, EBIE was performed using a 15keV beam at currents of 0.47-6 nA. A focused Gaussian electron beam was rastered over the region of interest and the irradiation process was recorded in real time. RESULTS Table 1 summarizes the range of substrates and gaseous environments used for EBIE experiments, the radicals created in each case, and the results of attempts at etching the Ag nanowires/substrate. The mechanistics of EBIE of hBN and diamond in oxidative environments has been described in detail in previous publications.16,

20.

H2O EBIE of hBN proceeds by

spontaneous dissociative chemisorption of H2O at defects and at the edges of hBN sheets, fragmentation of hBN into nanoscale hBN fragments, and subsequent volatilization of the nitrogen and boron constituents. Here, we find that efficient volatilization of both the hBN substrate and 3



the Ag nanowires occurs when the nanowires are in direct contact with the hBN substrate (Fig. 1a). Etching does not occur when the nanowires are dropcast on an oxide-covered Si substrate instead of hBN (Fig. 1b), or when EBIE is conducted in vacuum rather than in H2O (Fig. 1c). It is clear that this etching process is very different from Ag nanoparticle-catalyzed etching of hBN, which requires temperatures greater than 800 C25-26. Rather, the results shown in Fig. 1 indicate that EBIE of hBN in H2O produces a reactive species that is capable of highly efficient and localized chemical etching of Ag nanowires lying within the beam-irradiated region. (Note that the small degree of roughening of the Ag nanowires that can be observed in vacuum is due to the presence of residual H2O in the SEM chamber).

Table 1. Summary of substrate-precursor gas combinations used for EBIE, the radicals created, and results of Ag nanowire/substrate etch experiments. Substrate

Gaseous

Radicals present

Ag nanowire/substrate EBIE

environment hBN, AlN

H2O

OH*, O*, H*, N*

Both nanowires and substrate etched

hBN, AlN

O2

O*

Neither etched

hBN, AlN

NH3

H*, N*

Neither etched

Si

H2O

OH*, O*, H*

Neither etched

Si

O2 + N2

OH*, O*, N*

Neither etched

hBN, AlN

None (high vacuum)

None

Neither etched

GaN

H2O

OH*, O*, H*, N*

Neither etched

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GaN

None (high vacuum)

None

Neither etched

Figure 1. EBIE of Ag nanowires a) dropcast on hBN and irradiated in 8 Pa H2O, b) dropcast on Si and irradiated in 8 Pa H2O, c) dropcast on hBN and irradiated in high vacuum (scale bar=500 nm). In each case EBIE was conducted for 1 hour at a beam energy of 15 keV and beam current of 3.8 nA.

In order to determine whether etching of Ag nanowires occurs on other III-nitride substrates, H2O EBIE was also conducted on Ag nanowire-AlN and Ag nanowire-GaN heterostructures. As is the case for hBN, Ag nanowires on AlN are efficiently etched by H2O (Fig. 2 a, b). However, EBIE of Ag nanowires on a GaN substrate under 16 Pa H2O (Fig 3 a, b) does not result in any significant volatilization of either the Ag nanowires or the GaN substrate. Compositional analysis of the Ag-AlN and Ag-GaN samples was conducted by Energy Dispersive X-ray (EDX) spectroscopy at intervals during the H2O EBIE process (accurate EDX analysis was not possible for lighter elements such as B). EDX maps of Al, O and N were obtained from bare regions of the AlN and GaN substrate that had also been irradiated in H2O under similar conditions. Along with a slight decrease in Al content, the EDX maps reveal a reduction of N content and simultaneous 5



increase in O content in the beam irradiated region, indicating simultaneous denitrogenation and oxidation of the AlN substrate (Fig. 2 c-f). This is also revealed by the continuous drop in N/O ratio (from 0.90 to 0.22) during the EBIE process (Fig. 4). In the case of GaN, EDX analysis did not show any significant change in the N or Ga content in the e-beam irradiated region (Fig. 3 cf). This lack of volatilization of Ag/GaN heterostructures is most likely due to the formation of a stable gallium oxide layer on the GaN, which is apparent from the higher oxygen content in the irradiated region (Fig. 3d). As the thickness and surface coverage of the protective oxide layer increases, N* radical production stops, resulting in the observed plateau of the N/O ratio at ~0.75 after 90 min. irradiation (Fig. 4).

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Fig. 2: SEM images (a) before and (b) after 40 min. electron irradiation (15 keV, 3.8 nA) of Ag nanowires on AlN in 8 Pa H2O. The scale bar is 500 nm. (c-d) EDX mapping (3.5 keV, 1 nA) of an adjacent AlN region after 2 hrs EBIE (15 keV, 6 nA, 16 Pa) shows a decrease in Al and O content. (e-f) EDX mapping shows a slight decrease in N content after EBIE. The scale bar is 3 μm for all EDX maps.

Fig. 3. SEM images (a) before and (b) after H2O electron beam etching of Ag nanowires on GaN in 8 Pa H2O (scale bar=500 nm). Electron irradiation was performed for 40 mins at 15 keV and 3.8 nA. (c-d) EDS mapping (3.5 keV, 1 nA) of an adjacent region subjected to FEBIP for 2 hrs, 15keV 16 Pa, 6 nA) shows a decrease in Ga and increase in O content (scale bar=3 um). (e-f) EDS mapping performed before and after e-beam exposure reveal a slight decrease in N content after EBIE (scale bar=3 um). 7



Fig. 4. Change in N/O ratio during H2O EBIE of AlN and GaN. Compositional changes were calculated from EDX spectra taken after 0, 30, 60 and 90 minutes electron irradiation (15 keV, 6 nA) in 16 Pa H2O.

DISCUSSION In previous studies of oxidative and focused electron beam induced etching of hBN in H2O environments, spontaneous decomposition of H2O was found to begin at highly active edge and defect sites in the hBN20, 25, followed by volatilization of the hBN in the surrounding area. The 8


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EBIE process involves dissociation of precursor molecules upon surface adsorption via both spontaneous and electron-induced processes, resulting in the production of surface-adsorbed OH*, O* and H* radicals. These surface adsorbed radicals react with the sample to form volatile products, which are removed from the surface by electron stimulated desorption20, 27. Our experimental results (summarized in Table 1) demonstrate that simultaneous etching of the Ag nanowires and the nitride substrate in H2O is only observed when sufficient N* radicals are created via denitrogenation of the substrate by the focused electron beam. These N* radicals can then react with surface adsorbed OH*, O* and H* to form NO* in the following radical-radical reaction: N* + OH* → NO* + H* Reaction of the surface-adsorbed NO, H and O radicals then results in highly localized formation of nitric acid, HNO3: NO*+H*+2O* → HNO3 Nitric acid is known to etch Ag and other noble metals, and reacts with the boron component of the hBN to produce volatile boron compounds such as boric acid (a very weak acid that does not etch metals). The complete pathway leading to simultaneous electron beam induced etching of both Ag and hBN is shown in Fig. 5.

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Fig. 5. The proposed chemical reaction pathway leading to complete volatilisation of hBN during EBIE in a H2O environment. In aqueous solution, reaction of nitric acid with Ag results in the formation of the soluble nitrate AgNO3. Therefore to validate our proposed Ag and hBN volatilization mechanism, we conducted EBIE of AgNO3 in a low-pressure H2O environment using similar experimental conditions as those used for Ag-hBN etching in Fig 1a. Aqueous AgNO3 was dropcast onto a Si(111) substrate, and subjected to EBIE in 8 Pa H2O environment (Fig. 6a, b). The majority of the AgNO3 within the beam-irradiated region was volatilized after 60 min electron beam irradiation (Fig. 6b). EDS mapping of the irradiated area at the end of the EBIE process (Fig. 6 c-f) confirms the simultaneous removal of Ag, N and O species. Etching was not observed when AgNO3 was irradiated in high vacuum.

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Fig. 6. (a) Schematic of electron beam induced etching of AgNO3 dropcast and crystallized on a Si substrate in a H2O environment (8Pa). (b) SEM image of the etched region after a 60 min exposure (scale bar=200 nm) c-f) EDS mapping of the etched region for Ag, Si, O and N respectively. Electron beam irradiation was conducted at 15 keV and 0.47 nA and EDS mapping performed at 5 keV. CONCLUSIONS In conclusion, we have demonstrated a novel method of electron beam directed nanoscale chemical etching that utilizes radicals generated from beam-induced volatilization of the substrate for chemical etching of an overlying group 11 metal. Chemical etching of Ag nanowires dropcast on hBN and AlN substrates results from reactions between the N radicals generated by substrate volatilization and H and O radicals resulting from dissociation of H2O gas. The simplicity of this etch process and lack of sensitivity to precursor gas pressure and electron beam conditions (beam current and accelerating voltage) make it highly useful for nanoscale etching and patterning of noble metal/nitride heterostructures employed in plasmonic and photonic devices. This substrateassisted etch process could also be applied to etching of other materials that are etched by weaker acids than HNO3 (such as H3PO4 generated by EBIE of black phosphorus28). AUTHOR INFORMATION Corresponding Author Charlene Lobo, School of Mathematical and Physical Sciences, University of Technology Sydney, Ultimo, NSW, 2007, Australia E-mail: [email protected], Ph +61 9514 1673 Author Contributions

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Christopher Elbadawi and Johannes E. Fröch conducted the experimental work; Charlene Lobo, Christopher Elbadawi, Igor Aharonovich and Milos Toth designed the experiments and wrote the manuscript. ACKNOWLEDGMENT This work was partially funded by the Australian Research Council (project number LP170100150). We would like to thank Mika Tham and Kerem Bray for helpful discussions and assistance with graphics. REFERENCES 1.

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One Step Nanoscale Patterning of Silver Nanowire-Nitride

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