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Article Cite This: ACS Omega 2018, 3, 8036−8041
Hexagon Flower Quantum Dot-like Cu Pattern Formation during Low-Pressure Chemical Vapor Deposited Graphene Growth on a Liquid Cu/W Substrate Phuong V. Pham* SKKU Advanced Institute of Nano Technology (SAINT), SKKU, Suwon, Gyeonggi-do 440-746, Republic of Korea Center for Multidimensional Carbon Materials, Institute for Basic Science, Ulsan 44919, Republic of Korea Downloaded via 5.8.47.16 on July 26, 2018 at 00:36:36 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: The H2-induced etching of low-dimensional materials is of significant interest for controlled architecture design of crystalline materials at the micro- and nanoscale. This principle is applied to the thinnest crystalline etchant, graphene. In this study, by using a high H2 concentration, the etched hexagonal holes of copper quantum dots (Cu QDs) were formed and embedded into the large-scale graphene region by low-pressure chemical vapor deposition on a liquid Cu/W surface. With this procedure, the hexagon floweretched Cu patterns were formed in a H2 environment at a higher melting temperature of Cu foil (1090 °C). The etching into the large-scale graphene was confirmed by optical microscopy, atomic force microscopy, scanning electron microscopy, and Raman analysis. This first observation could be an intriguing case for the fundamental study of low-dimensional material etching during chemical vapor deposition growth; moreover, it may supply a simple approach for the controlled etching/growth. In addition, it could be significant in the fabrication of controllable etched structures based on Cu QD patterns for nanoelectronic devices as well as in-plane heterostructures on other low-dimensional materials in the near future.
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INTRODUCTION Graphene, a honeycomb crystal carbon lattice, has attracted huge research interest during the past few years due to its anomalous properties, including very high carrier mobility, extremely high mechanical strength and optical transparency, electrical conductivity, etc.1−25 As a result, graphene is considered to be an ideal nanomaterial for next-generation semiconductors to replace silicon. The etching of material is the block removal from a material matrix by chemical or physical methodsthe reverse of the growth process. Understanding the etching mechanism is necessary for material design as well as the realization of its capabilities. Material growth/etching requires a high-energy barrier nucleation process. Controlling the etching/growing parameters may originate from the formation of a thermodynamic and stably ordered structure with the ability to form a different kinetically controlled metastable structures with high-energy crystal facets and edges. The material growth systems have been controlled with different scales (nanometer to micrometer).26−28 The family of snow-crystal-like graphene with patterns is formed by a nonlinear process in nature.29 In contrast, the highly crystalline materials (e.g., Si) are etched via an anisotropic rule,30 leading to stable etched patterns with Euclidean geometries. The underlying mechanism is attributed to various etching rates on various crystalline directions and surfaces related to various free energies. © 2018 American Chemical Society
However, the etched structures of low-dimensional materials beyond Euclidean geometries have not been investigated. The high crystal C atom single-layer supplies a simple model to study the fundamental growth and/or etching process via the chemical vapor deposition (CVD) method. Significant efforts have been created to develop graphene growth strategies with controlled size, crystallinity, and edge structures via CVD.31−34 In addition, several reports on graphene etching have been carried out utilizing various etchants, such as plasma H2,35−38 H2,39,40 and metallic nanoparticles.41−43 Recently, graphene crystal patterns were well-controlled through inert gases and a H2 source.44 Those discoveries inspired us to additionally examine the fundamental issue of the graphene etching mode. In this work, we report the first observation of a new kind of etched geometry of a large-scale grown graphene region that is beyond known Euclidean geometries to date. The hexagon quantum dot (QD)-like etched Cu pattern is a new morphology that has not yet been revealed.
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RESULTS AND DISCUSSION For the etched graphene growth procedure on a liquid Cu/W substrate, a schematic of the hexagon QD-like Cu pattern Received: May 12, 2018 Accepted: July 9, 2018 Published: July 18, 2018 8036
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Figure 1. (a) Schematic of the formation of a hexagon flower QD-like Cu pattern during low-pressure CVD graphene growth on a liquid Cu/W substrate.
Figure 2. (a) OM image of bare Cu foil (thick 250 μm). (b) SEM image of liquid Cu at 1090 °C after being resolidified. (c) Micrograph photo of etched large-scale graphene on liquid Cu/W. (d) AFM of liquid Cu on W foil after being resolidified. (e) Thickness of liquid Cu at 1090 °C and resolidified at about 150 nm, corresponding to the blue line in (d). (f) OM images of liquid Cu on W foil after being resolidified, corresponding to the AFM image in (d).
Figure 3. OM images of hexagon flower-etched Cu patterns embedded into the large-scale graphene located on a liquid Cu/W substrate. 8037
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Figure 4. SEM images of hexagon flower-etched Cu patterns embedded into the large-scale graphene located on a liquid Cu/W substrate at different magnifications.
formation during low-pressure CVD graphene growth onto a liquid Cu/W substrate is illustrated in Figure 1. Low-pressure chemical vapor deposition (LPCVD) is investigated at low pressure (6 Torr) using Cu (thickness of 250 μm) as a catalytic substrate located on a W foil (thickness of 80 μm). First, a Cu/W substrate was heated in the H2 environment (100 sccm) for 30 min to the melting point of Cu (1090 °C) and then annealed in H2 (100 sccm) for 30 min. The integrated etching/growth proceeded via the LPCVD approach with hexagon flower-etched Cu patterns, embedded into the large-scale graphene located on a liquid Cu/W substrate using a CH4/H2 ratio of 6/100 sccm in 30 min. Finally, the furnace was opened and CH4 gas was turned off; H2 flow was continued for fast cooling to 100 °C. There have been several investigations on the controlled diffusive etching modes for crystal growth of low-dimensional materials that are responsible for etched graphene pattern formation,1−4,39 generating an effective engineering method for etched patterns on other 2D and 3D material systems. However, etching to form flower QD-like Cu patterns embedded into the large-scale graphene region is quite new, strange, and intriguing in the field of etching at the micro- and nanoscale. To date, there is no scientific report in terms of theory and simulation investigations that detail the relationship of the hexagonal-shaped etching mode and the underneath graphene structure. For morphology investigations of solid Cu, liquid Cu, and graphene before and after the CVD process, scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical microscopy (OM) were carried out, as shown in Figure 2a−f. Here, Figure 2a exhibits the morphology of bare Cu foil (250 μm) through the SEM image before the CVD process. Figure 2b reveals the SEM image of liquid Cu at 1090 °C after being resolidified. Figure 2c is the micrograph photo of etched large-scale graphene on the liquid Cu/W substrate. Figure 2d reveals the AFM image of the surface liquid Cu after being resolidified on W foil with a liquid Cu thickness of 150 nm after 1090 °C and resolidified as shown in Figure 2e, corresponding to the blue line in Figure 2d. Figure 2f is the OM images of liquid Cu located on W foil after being resolidified, corresponding to the AFM image in Figure 2d. Figure 3 and Figures S1 and S2 in the Supporting Information reveal a new Euclidean geometry of the QD-like Cu patterns which are partially etched Cu hexagons at the center of six edge sites and embedded into the large-scale graphene located on
Figure 5. AFM images of hexagon flower-etched Cu patterns embedded into large-scale graphene located on a liquid Cu/W substrate, (a,b) 3D AFM mapping image, (c) 3D AFM mapping image at different views of (a), and (d) phase image of (a).
the liquid Cu/W surface after a higher melting point of Cu foil (1090 °C). Because the melting temperature for W foil was 3422 °C, its morphology did not change after treatment at 1090 °C. Similarly, the SEM images at various magnifications were taken for hexagon flower-etched Cu patterns embedded into large-scale graphene located on a liquid Cu/W substrate, as shown in Figure 4 and Figures S3 and S4 in the Supporting Information. For further demonstrations, the AFM data were applied to observe the hexagon flower-etched Cu patterns embedded into large-scale graphene located on a liquid Cu/W substrate, as shown in Figure 5 and Figure S5 in the Supporting Information for two-dimensional (2D) and three-dimensional (3D) images at different corners. In addition, their phase images, which are characterized for softness, stiffness, and the adhesion between the AFM cantilever tip with the specimen surface, are shown in Figure 5d and Figure 5Sd. 8038
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Figure 6. (a,b) OM images captured from Raman spectroscopy of hexagon flower-etched Cu patterns embedded into the large-scale graphene located on a liquid Cu/W substrate. (c,d) Raman data of different positions marked as blue, green, and red crosses in (a,b). (e,f) Zoomed-in image of graphene in (c,d), respectively. (g) Raman mapping with a scan range from 0 to 800 nm of region (b).
To investigate the existence of graphene on liquid Cu/W and the Cu pattern etching effect, Raman spectroscopy was carried out at two regions: graphene and Cu pattern (see Figure 6). At the Cu pattern regions, the Raman peaks in Figure 6c,d correspond to the OM images that are directly captured by the Raman analysis instrument, with the red cross in Figure 6a and the black cross in Figure 6b. They showed the presence of some peaks of CuO, Cu2O, and Cu(OH)2, which are already well-known from a previous report.45 On the other hand, in the blue and green graphene regions in Figure 6c and the red region in Figure 6d, corresponding blue and green crosses in Figure 6a and red cross in Figure 6b show clearly the D, G, and 2D peaks of Raman data as the fingerprints of the graphene structure on liquid Cu/W substrates. The Raman mapping of Figure 6b was also obtained for the etched Cu pattern at a wavelength scan range from 0 to 800 nm for the typical existence of CuO, Cu2O, and Cu(OH)2 peaks (Figure 6g), which was proven in a previous report.45 Figure 6a,f shows the zoomed-in images of Figure 6c,d, respectively. For an explanation about the mechanism of etched Cu pattern formation embedded into large-area graphene film, see Figure 7. It is believed that the etching mode occurred due to
Figure 7. Mechanism of etched Cu patterns embedded into the largearea graphene film with diffusion of etchants (H2 molecules or H radicals) underneath and above the liquid Cu surface, which could be mostly responsible for the formation of etched Cu patterns.
the diffusion of etchants (H2 molecules or H radicals) at the graphene/liquid Cu interface and the diffusion on the graphene surface. Consequently, the etching effect formed at the defects sites or grain boundaries was embedded into the 8039
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large-scale graphene to create the hexagon QD-like Cu patterns. Here, the diffusion at the graphene/liquid Cu interface might be the key role for formation of the etched Cu pattern. The high etching rate is because of high H2 etchant concentration as well as hindered diffusion at the graphene/liquid Cu interface. H2 diffusion on isotropic liquid Cu indicates the physical origin of the high symmetry of etched Cu patterns. Also, it is a key factor for visualizing the etched line mode because controlled diffusive etching varies with nanoparticle etching from previous reports.41−43
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CONCLUSIONS The resulting hexagon flower QD-like etched Cu patterns have revealed a new kind of unknown Euclidean geometry during large-scale CVD graphene growth. Here, we have established the first observation of etched Cu patterns on large-scale graphene that can form a new etched hexagon shape. The experimental results provide clear proof of this etching mode. Etching to form the hexagon QD-like Cu pattern was induced at high H2 concentration. This study is expected to be further computationally and experimentally examined for in situ observation of integrated graphene etching/growth in the future. This new integrated growth/etching effect shows an unknown graphene etching mode that enables other nanomaterial structures to be formed. In addition, further investigation of the graphene etching mode on new substrates (e.g., Ni/W) is another intriguing phenomenon which will be discovered in the near future.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00985. OM, SEM, and AFM images of hexagon flower QD-etched patterns embedded into the large-scale graphene located on the liquid Cu/W foil (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Phuong V. Pham: 0000-0001-7951-1329 Notes
The author declares no competing financial interest.
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ABBREVIATIONS OM, optical microscopy; SEM, scanning electron microscopy; AFM, atomic force microscopt; QDs, quantum dots; CVD, chemical vapor deposition; LPCVD, low-pressure chemical vapor deposition; 2D, two-dimensional; 3D, three-dimensional
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REFERENCES
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