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Nanowire-on-Nanowire: All-Nanowire Electronics by On-Demand Selective Integration of Hierarchical Heterogeneous Nanowires Habeom Lee,†,□ Wanit Manorotkul,†,□ Jinhwan Lee,‡ Jinhyeong Kwon,† Young Duk Suh,† Dongwoo Paeng,⊥ Costas P. Grigoropoulos,⊥ Seungyong Han,§ Sukjoon Hong,*,∥ Junyeob Yeo,*,¶ and Seung Hwan Ko*,†,# †
Applied Nano and Thermal Science Lab, Department of Mechanical Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea ‡ Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States ⊥ Laser Thermal Lab, Department of Mechanical Engineering, University of California, Berkeley, California 94720, United States § Department of Mechanical Engineering, Ajou University, San 5, Woncheon-Dong, Yeongtong-Gu, Suwon 16499, Korea ∥ Department of Mechanical Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Korea ¶ Novel Applied Nano Optics Lab, Department of Physics, Kyungpook National University, 80 Daehak-ro, Pook-gu, Daegu 41566, Korea # Institute of Advanced Machinery and Design (SNU-IAMD), Seoul National University, Gwanak-ro, Gwanak-gu, Seoul 08826, Korea S Supporting Information *
ABSTRACT: Exploration of the electronics solely composed of bottom-up synthesized nanowires has been largely limited due to the complex multistep integration of diverse nanowires. We report a single-step, selective, direct, and ondemand laser synthesis of a hierarchical heterogeneous nanowire-on-nanowire structure (secondary nanowire on the primary backbone nanowire) without using any conventional photolithography or vacuum deposition. The highly confined temperature rise by laser irradiation on the primary backbone metallic nanowire generates a highly localized nanoscale temperature field and photothermal reaction to selectively grow secondary branch nanowires along the backbone nanowire. As a proof-of-concept for an all-nanowire electronics demonstration, an all-nanowire UV sensor was successfully fabricated without using conventional fabrication processes. KEYWORDS: laser-induced hydrothermal growth, localized heating, heterogeneous nanowires, hierarchical nanostructures, ZnO nanowire, silver nanowire that covers core components of electronics,1,7 and the development of a low-temperature solution synthesis process including polyol and hydrothermal methods further lowers the barrier to entry by removing the need for complex equipment and harsh environments required for NW synthesis.8−13
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hemically synthesized nanowires (NWs) are excellent building blocks for nanoelectronics. They inherently possess not only a sub-micrometer feature size that is challenging to achieve even with the most state-of-the-art photolithography techniques1,2 but also an atomically smooth and ordered crystalline structure, which enables superb electrical properties together with high mechanical robustness as confirmed from numerous studies.3−6 At the same time, extensive research on NWs to date has stretched the range of chemically synthesizable NWs from metal to semiconductor © 2017 American Chemical Society
Received: August 27, 2017 Accepted: October 27, 2017 Published: October 27, 2017 12311
DOI: 10.1021/acsnano.7b06098 ACS Nano 2017, 11, 12311−12317
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Figure 1. (a) Schematic of the LIHG process on a single Ag NW. A visible laser is focused at a specific spot on the backbone Ag NW, and a confined temperature field is generated with the laser focus at the center. As the temperature rises above the threshold temperature, the growth of ZnO NW is initiated from the ZnO QD functionalized on the Ag NW, and the growth continues only within the laser heating spot. (b) Illustration of the resultant hierarchical heterojunction of ZnO NW branches on the Ag NW backbone. (c) SEM image of the hierarchical heterojunction of ZnO NW branches on a Ag NW backbone that is suspended on an etched Si substrate. It is noticeable that the ZnO NW is grown selectively on the suspended Ag NW by the LIHG process.
of the target backbone NW only.28 However, the Joule heating method fails in site-selective synthesis of an NW along its axial direction because whole the nanowire was heated, and its application is often more complex due to the need of direct electrical connection to the external power supply. A more advanced approach is followed by utilizing a laser-induced photothermal reaction that enables remote generation of a temperature field with high precision and controllability.26,27 The strength of the laser growth has been confirmed through applying the process to arbitrary substrates including 3D structures and tailored absorption layers; however, no successful validation has been achieved to date for a nanoscale selective growth. In this research, we report the demonstration of on-demand selective laser integration of secondary heterogeneous branched metal oxide NWs on a primary backbone metal NW in a highly selective manner based on a heater-assisted laser-induced hydrothermal growth (LIHG) process. The most widely studied NWs, silver (Ag) and zinc oxide (ZnO) NWs, are selected as primary backbone and secondary branch NWs
From all these characteristics of NWs and their development status, nanoelectronics solely composed of chemically synthesized NWs, or all-nanowire electronics, with metal NW electrodes and functional semiconductor NW channels, appears to be an attractive form of future electronics.14−16 Regarding the subject, precise placement or assembly of diverse NWs at a specific place is a first step as well as the key technology to realize such nanoelectronics, yet remained as a highly challenging task even with a single type of NW.17−21 As a consequence, other approaches have been investigated for the preparation of NWs since the usage of chemically synthesized NWs has been largely restricted.22−24 For more practical integration of NWs for nanoelectronics, separate steps of synthesis and placement/assembly should be combined. Therefore, selective and direct synthesis of NWs in a precursor environment has been proposed recently.25−28 A localized temperature field required for confined NW growth is typically achieved by an electrically driven Joule heater,25,28 and its size has been successfully reduced down to a single NW in the latest research to obtain heterogeneous NWs in the vicinity 12312
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Figure 2. (a) SEM image of ZnO NW arrays grown on four distinct spots (denoted as red dotted circles) along a single Ag NW. Each spot is exposed to 10 min of laser irradiation. (Inset) Tilted SEM image of the ZnO NW on the Ag NW. (b) ZnO NW array size grown on the Ag NWs with different diameters according to the laser irradiation time. (c) Magnified SEM picture of a ZnO NW array with a hexagonal cross section. (Inset) TEM image of the ZnO NW array.
Supporting Information S1. Since the threshold temperature for ZnO NW growth is known to be ∼60 °C,30 the heat flux from the electrical heater is controlled by a feedback circuit to maintain the temperature of the precursor solution at 50 °C throughout the study. With the conventional LIHG process, as the size of a focused laser is limited down to several micrometers at visible wavelength, the region of heat generation from the photothermal reaction is controlled at the same scale as well.26,27 Because the 1D nanomaterial is both nanoscale in the diameter direction and microscale in the length direction, the laserheated spot size can be controlled even smaller than the laserfocused size in the diameter direction. As the laser-induced heat is generated at a designated point within the target Ag NW, the temperature of the surrounding precursor solution increases through conductive and convective heat transfer and immediately overcomes the threshold temperature for ZnO NW growth. Although the local heating occurs nearly instantaneously, the outward growth of the branch ZnO NWs from the backbone Ag NW is highly confined since the temperature decreases rapidly along the radial direction to set a controlled growth thermal activation zone, as shown in the inset of Figure 1a. The generated heat dissipates through the Ag NW at the same time, yet the selectivity of the growth in the axial direction is preserved owing to the limited thermal conductivity of the Ag NW.31 As a result, a ZnO NW is grown selectively on the Ag NW to yield a hierarchical ZnO NW on the Ag NW heterojunction on-demand as conceptually shown in Figure 1b. At the current optical configuration, a 532 nm continuous wave laser at 1 W power is focused with a 5× objective lens as a primary heat source to yield a focused spot of ∼10 μm diameter. Detailed information on the optical setting can be found in the Supporting Information S2. The peak intensity at the focus reaches as high as ∼2.55 × 1010 W/m2, yet the amount of laser-induced selective heating is expected to be small since the absorption cross section of a silver nanowire per unit length is on the order of a nanometer, and it is expected that the total heat generation from the laser is on the order of a milliwatt. Supporting Information S3 illustrates the resultant
throughout this study. In order to overcome the small absorption cross section of the backbone Ag NW, a hybrid scheme using a heater elevating the background temperature immediately below the threshold growth temperature of the secondary branch ZnO NW with polarization-dependent light absorption of the Ag NW is investigated to achieve hydrothermal growth of ZnO NWs at relatively moderate laser intensity. By combining these two schemes, a ZnO NW is selectively grown at the desired spot on a Ag NW to accomplish direct selective integration of metal and metal-oxide NWs ondemand. A UV sensor based on the proposed selective lasergrown ZnO NW branch on a Ag NW backbone is further fabricated as the simplest form of proof-of-concept for allnanowire electronics and demonstrates its potential as future electronics with compact size, low power consumption, and fast response.
RESULTS AND DISCUSSION Figure 1a shows the schematic illustration of the on-demand digital laser integration of heterogeneous NWs. The proposed scheme is based on a laser-induced hydrothermal growth process,26,27 and the laser was irradiated on the 1D single metallic nanowire as a laser absorption layer. Since Ag NWs are synthesized through a wet chemical method in a separate step,29 the absorbing layer is prepared by simple deposition of a Ag NW on a glass substrate, followed by functionalizing the surface with a ZnO quantum dot (QD) solution for seeding.12,27 A detailed explanation on the sample and chemical preparations can be found in the Experimental Section. As the laser is focused on the ZnO nanoparticle functionalized Ag NW immersed in precursor solution, a photothermal reaction occurs only at the laser-irradiated spot acting as a primary heat source, yet it should be noted that the absorption cross section of a 1D single Ag NW is extremely small compared to the 2D or 3D absorbing layers utilized in previous studies.26,27 We therefore introduce an electrical heater as a hybrid secondary heat source to elevate the background temperature slightly below the threshold temperature for ZnO NW growth so that the laser intensity required to trigger the ZnO NW growth can be reduced. The overall experimental setup can be found in 12313
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Figure 3. (a) Laser polarization dependence. SEM image of the ZnO NW arrays grown on a single Ag NW with different laser polarization. (Insets) Magnified SEM images of the ZnO NW arrays grown with the laser polarization at (i) 45°, (ii) 60°, and (iii) 90° to the axis of the underlying Ag NW. Each spot is exposed to 6 min of laser irradiation. (b) ZnO NW array size grown on the Ag NWs with different diameters according to the laser polarization. (c) FDTD simulation on the electrical field intensity profile in the vicinity of the Ag NW with a pentagonal cross section when the polarization of the incoming laser is parallel (×) or perpendicular (↔) to the NW axis.
temperature field estimated by numerical simulations with a conjugate convective heat transfer model when the suspended Ag NW in water is under constant heat generation of 3 mW while the background temperature is maintained at 50 °C. The simulation does not reflect the complete actual process including chemical reactions and optical disturbances that might occur during the growth; the result suggests that a small amount of heat generation from the Ag NW is sufficient to meet the elevated temperature required for ZnO NW growth. Due to the sharp temperature gradient from the backbone NW, natural convection occurs around the Ag NW at a fraction of μm/s, and this value is in accordance with a previous study on the laser-induced convective current in water without boiling.32 A representative example of the hierarchical heterojunction of ZnO NWs on a suspended Ag NW fabricated by the proposed laser process is shown in Figure 1c. For the confirmation of isotropic radial growth of a ZnO NW from the backbone Ag NW, Ag NWs are first suspended on a silicon wafer with predefined microwells, and the Ag NW with a relatively thick diameter (>300 nm) is chosen as the target backbone NW in order to prevent stiction problems, which can be problematic for thinner Ag NWs, as in the Supporting Information S4. It is observable that the area subject to the ZnO NW growth closely resembles the isothermal contour plots from the numerical simulation result, proposing that the growth of the ZnO NW branches is mainly determined by the laser-generated temperature field. Hierarchical heterojunctions between two different types of NWs are frequently reported;33−35 however, the locationselective growth of secondary branch NWs in the axial direction of the backbone NW has not been achieved. Figure 2a shows ZnO NW arrays grown at four distinct sites on a single Ag NW backbone deposited on a glass substrate through sequential applications of the proposed laser process with a 10 min irradiation time for each spot. It is apparent from the figure that the heterojunction hierarchical ZnO NWs on a single Ag NW marked with red dotted circles are separated from each other
and clearly distinguished from the bare Ag NW. Therefore, we achieve a method that integrates two heterogeneous chemically synthesized NWs in a selective and direct manner without any photolithography techniques. The resultant of ZnO NW array after complete growth resembles a 1D confined hemispherical sea urchin shape along the axis of the Ag NW, as shown in the inset figure in Figure 2a. In this study, only Ag NWs with long length (>100 μm) and large diameter (>100 nm) are selected as the target backbone NWs to provide enough space between selective ZnO NW growth spots in the axial direction and ensure a clear dark-field image in an in situ optical vision system for precise localization of the laser irradiation along the same Ag NW. The grown ZnO NW array sizes dependent on laser irradiation time are plotted in Figure 2b for various backbone NW diameters. The lateral size of the ZnO NW array is approximately proportional to the laser irradiation time in the early stage (10 μm/h) is significantly faster than the growth rate of the conventional bulk growth (∼2 μm/h). A similar trend has been observed in other selective NW growth techniques at the microscale as well,27 and possible reasons behind this phenomenon include enhanced ion supply in the precursor solution for NW growth due to the enhanced natural convection or diffusion, negligible consumption in overall ion concentration, and superheating stemming from a very tiny heating volume. Meanwhile, no strong correlation between diameter of the Ag NW and lateral array size of the resultant ZnO NW is found from the results because of the complexity of the overall process that involves numerous variables such as heat conductivity/capacity corresponding to each diameter of the Ag NW and conduction/convection between Ag NW, glass, and precursor solution. The high-magnification scanning electron microscope (SEM) image in Figure 2c shows that 12314
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Figure 4. (a) Schematics for the fabrication of the proof-of-concept all-nanowire UV sensor. (b) SEM image of the resultant all-nanowire UV sensor. (Inset) Magnified SEM images of the ZnO NW arrays with pseudocolor. It is observable that the ZnO NWs grown from two separate Ag NWs are brought in contact to form a photosensitive Ag NW−ZnO NW heterogeneous network. (c) I−V curves of the Ag NW−ZnO NW heterogeneous network UV sensor with and without UV illumination. (d) Photocurrent measurement with switching UV illumination under 0.1 V bias.
the laser-grown ZnO NW after 10 min of irradiation has a hexagonal cross section, which verifies its crystalline structure together with the TEM image in the inset as reported in previous studies.36 When a subwavelength structure is utilized as a laserabsorbing layer, it is required to control the laser polarization minutely since the effective laser absorption is strongly dependent on the incident laser beam polarization direction.37 Figure 3a together with the insets shows the SEM images of ZnO NW array growth on a Ag NW by the LIHG heating process at 1 W laser power for 6 min each, but with different polarization angles of 45°, 60°, and 90° with the axial direction of the backbone Ag NW. The size of the secondary ZnO NW branch array grown on the Ag NW is apparently increased as the laser polarization becomes perpendicular to the Ag NW. Also, with various diameters (from ∼180 to ∼500 nm) of Ag NW, the grown ZnO NW branch array sizes consistently increase in every case as the laser polarization becomes more perpendicular to the target Ag NW, as plotted in Figure 3b. The lateral size of the ZnO NW array can be elongated as much as 2.5 μm when the Ag NW is irradiated with the laser at perpendicular polarization for 6 min. The laser polarization effect can be explained with the polarization-dependent laser absorption using numerical simulations in Figure 3c. Numerical simulation is conducted with a commercial program (Lumerical) to investigate the laser polarization effect of a 532 nm wavelength laser on a Ag NW with pentagonal cross section having a radius of 100 nm. Inferring from the simulation result, it is expected that electrical field enhancement is maximum when the polarization direction is orthogonal (TE mode) to the axis of the Ag NW. The enhancement is from the surface plasmon excitation of the Ag NW, which is initiated
from the upper edges and propagates toward the lower edges of the Ag NW,38 as can be confirmed from the intensity profile. The diameters of the Ag NWs are diverse in our study, and their edges can be not as sharp as the one replicated in the simulation, yet our simulation with a circular cross section of various diameters confirms that such a trend is still valid for wide ranges of conditions as well (Supporting Information S5). A ZnO NW has been widely employed as a UV sensor; however, an all-nanowire-based UV sensor solely composed of chemically synthesized NWs has not been reported. As a proofof-concept, an all-nanowire UV sensor is created by applying the proposed on-demand selective hierarchical heterogeneous integration by the LIHG process on two adjacent Ag NWs so that the ZnO NW grown from the Ag NWs can be connected as a photoconductive channel network as schematically shown in Figure 4a. For the characterization, electrical pads are prepared at each end via the laser sintering scheme reported previously without using any conventional photolithography or vacuum deposition.39,40 The SEM image in Figure 4b together with the pseudocolored inset indicates that the functional NWs from two backbone Ag NWs are well connected after applying the LIHG process for 10 min each. The measurements are conducted with a 365 nm wavelength UV lamp at 300 μW/cm2 power (ENF-250C, Spectroline) under ambient conditions using a probe station in a dark Faraday cage. The I−V characteristics of the fabricated UV sensor according to UV illumination are illustrated in Figure 4c. In this study, the difference in conductance with regard to the UV illumination can be observed with a very small bias voltage, while its I−V curve shows asymmetric behavior.41 The conductance is measured at relatively low bias voltage since the Ag NW can be damaged by a high voltage. Figure 4d is the switching 12315
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ambient conditions. The sample is then rested at 150 °C for 30 min to completely evaporate the polyol coated on the surface of the Ag NWs. The ZnO QD seeds are then deposited onto the Ag NWs by wetting the ZnO QD seed solution. The sample is immersed in ZnO precursor, and the binary LIHG heating process is applied. LIHG Process and Its Experimental Setup. As depicted in Figure S1, the experimental setup comprises a 532 nm Nd:YAG laser, which is used for laser heating as the localized heat source, and a thermal heater as an additional heating source for the entire system. For in situ observation of the process and precise manipulation of the focused laser spot, CCD cameras are located on both sides. The objective lens (OBJ1) used in this setup has a 5× magnification, providing a laser spot of ∼10 μm diameter at the focal plane. A half/ quarter-wave plate and a polarizer (red and blue rectangular units) are installed for the manipulation of the laser polarization. Background Heating. Background temperature is a delicate variable in this study, since the elevated background temperature of the ZnO precursor can nucleate homogeneous ZnO particles as well as grow unwanted ZnO NWs. On the other hand, the background temperature should be sufficiently high so that the laser intensity can be minimized. In this study, the background temperature is controlled to be 50 °C to assist the laser heating. It is confirmed that no ZnO NW growth is observed in >1 h duration at this heating condition.
photocurrent measurement in accordance with the existence of UV illumination under ambient conditions with a 0.1 V external bias. The current measured with the UV illumination is around 0.7 nA, and this value is much higher than the dark current, which is under 0.3 nA. Although the on/off ratio is small, the photocurrent rise and decay times are considerably fast compared to the other UV sensors with a similar configuration.27,42
CONCLUSION In this study, we report a next-generation all-nanowire electronics fabrication method using an LIHG-based ondemand laser process for facile and selective integration of heterogeneous hierarchical NWs overcoming the limitation of the conventional LIHG process due to the limit of the focused laser spot size. The highly confined temperature field was generated on the metallic nanowire, and subsequently thermochemical reaction occurred only at the confined temperature field. Although heterogeneous NWs are reported frequently elsewhere, we achieve spatial selectivity of branched secondary NW growth in the axial direction of the backbone NW without using any conventional photolithography or vacuum deposition. It is further demonstrated that the resultant heterogeneous NW structures, unlike those prepared through top-down approaches, well possess the advantages of bottomup synthesis as confirmed from their crystalline structures. As the simplest form of a proof-of-concept for all-nanowire electronics, a UV sensor based on the proposed selective laser grown ZnO NW branch on a Ag NW backbone is further fabricated and demonstrates its potential as future electronics with compact size, low power consumption, and fast response. The primary backbone and the secondary branch NWs have been mainly demonstrated for ZnO and Ag throughout the current study; however, we expect that the proposed process can be further expanded to other material combinations whose hydrothermal growth routes are known. With a broader range of applicable NWs, the proposed process shows great promise in the bottom-up fabrication of all-nanowire nanoelectronics, such as multifunctional environmental sensors.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b06098. Experimental setup of LIHG, results of COMSOL simulation, stiction problem for thin Ag NWs, and results of FDTD simulations (PDF)
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Seung Hwan Ko: 0000-0002-7477-0820 Author Contributions □
Habeom Lee and Wanit Manorotkul contributed equally to this work.
EXPERIMENTAL SECTION Chemical Preparation. A single-crystalline Ag NW, which is used as the selective laser energy absorbing material for the LIHG process, is prepared by the modified polyol synthesis for extralong size. The average length of the resultant Ag NW is up to ∼300 μm with a diameter ranging from 100 to 400 nm. The diameter of the Ag NWs used in this study is >200 nm for facile visualization in the CCD vision system. For the preparation of ZnO QD seeds, 30 mM sodium hydroxide (NaOH, Sigma-Aldrich) in 30 mL of ethanol is carefully dropped into 10 mM zinc acetate dehydrate (Zn(OAc)2, Sigma-Aldrich) in 60 mL of ethanol solution to prevent generation of white precipitates. The mixed solution is then heated with vigorous stirring for 2 h at 60 °C and cooled to room temperature. For the ZnO NW precursor, 25 mM zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Sigma-Aldrich) and 25 mM hexamethylenetetramine (HMTA, C6H12N4, Sigma-Aldrich) are mixed into 100 mL of DI water. After that, 5−7 mM polyethylenimine (PEI, C2H5N, SigmaAldrich) is added to the mixture. The resultant solution is heated at 95 °C for 1 h and cooled to room temperature. Lastly, the white precipitates are removed by a filtration process. Sample Preparation. Except the one shown in Figure 1c, a clean glass substrate, which is transparent in the visible region, is used in this study. An as-synthesized Ag NW solution is deposited onto the substrate with the assistance of a fluidic channel and allowed to dry in
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work is supported by the National Research Foundation of Korea (NRF) grant funded through the Basic Science Research Program (2017R1A2B3005706, NRF-2016R1A5A1938472, NRF-2016R1C1B1014729, NRF-2017R1C1B1008847), Institute for Information & Communications Technology Promotion (IITP) grant funded by the Korean government (MSIP) (No. 2017000910001100), and the Creative Materials Discovery Program (NRF-2016M3D1A1900035). REFERENCES (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. One-Dimensional Nanostructures: Synthesis, Characterization, and Applications. Adv. Mater. 2003, 15, 353−389. (2) Yang, P.; Yan, H.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R.; Choi, H. J. Controlled Growth of ZnO Nanowires and Their Optical Properties. Adv. Funct. Mater. 2002, 12, 323−331. 12316
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DOI: 10.1021/acsnano.7b06098 ACS Nano 2017, 11, 12311−12317
