大连理工大学《Small Methods》：一种协同转移法！实现大面积石墨烯清洁低缺陷转移

石墨烯具有优异的电学、光学与力学性能，是下一代电子器件的重要候选材料。化学气相沉积（CVD）可在铜箔上生长英寸级单层石墨烯，但制备器件前，必须把原子级厚度的石墨烯从生长金属转移到绝缘或柔性衬底上。然而，最常用的聚甲基丙烯酸甲酯（PMMA）支撑转移容易引入裂纹、褶皱和聚合物残留，还会带来非预期掺杂，明显损害器件性能。如何在大面积转移中同时做到低损伤与高洁净度，仍是石墨烯走向应用的关键难题。基于此，本文，大连理工大学 李经民教授团队在《Small Methods》期刊发表题为”Interfacial Copper Oxidation Under an Applied Alternating Electric Field and a Water-Soluble Caramel Support Enable Clean, Low-Defect Transfer of Large-Area Graphene”的研究论文。该工作把交变电场下的界面铜氧化与水溶性焦糖支撑结合起来，提出了一种协同转移策略：在 50 Hz、名义场强 300 V/mm 的交变电场中处理 5 h，使石墨烯/铜界面形成富氧化亚铜（Cu₂O）层，随后用焦糖支撑层进行剥离与转印。

此外，由蔗糖与玉米糖浆制成的焦糖支撑层玻璃化转变温度约 58–69 °C，加热至 70 °C 即可热塑软化，与多种目标衬底共形贴合；它与石墨烯作用弱，转移后可用水直接去除，避免了 PMMA 类聚合物残留。经表征，在相同的液相处理条件下，交变电场处理的石墨烯/铜界面氧化 1 h 即达约 70%、约 3 h 超过 90%，而纯水对照处理12h仅约 60%；纹理分析对比度由 0.25 降至 0.14，说明氧化更快也更均匀，界面形成富 Cu₂O 状态，石墨烯与铜的黏附显著减弱，便于低损伤剥离。利用该方法实现了 4 英寸石墨烯向 SiO₂/Si 等衬底的转移，所得薄膜表面均方根（RMS）粗糙度仅 0.73 nm。本研究提出了一种将界面氧化剥离与水溶性焦糖支撑相结合的协同转移路线，为大面积石墨烯的清洁、低缺陷转移提供了一条可扩展、工艺温和、支撑材料廉价且可水洗去除的新途径，有望推动高保真图形化与规模化器件集成。

图1、Schematic illustration of interfacial oxidation of graphene/Cu under an applied alternating electric field and subsequent caramel-assisted graphene transfer.

图2、Interfacial oxidation of graphene/Cu under water-only and alternating-field conditions. (a) Schematic illustration of the alternating-field treatment configuration. (b) Representative optical micrograph of graphene/Cu after water-only treatment for 12 h and (c) the corresponding GLCM analysis. (d) Time-dependent oxidized-area fractions under water-only and alternating-field conditions. (e) Representative optical micrograph of graphene/Cu after alternating-field treatment for 5 h and (f) the corresponding GLCM analysis. (g–i) Reference-spectrum-based NLLSF fits of Cu LMM Auger spectra, (j–l) high-resolution Cu 2p XPS spectra, and (m–o) peak-fitted high-resolution O 1s XPS spectra of pristine, water-treated (12 h), and alternating-field-treated (5 h) graphene/Cu, respectively.

图3、Interfacial and thermomechanical principles of caramel-assisted graphene transfer. (a) Surface energies derived from measured contact angles on different surfaces (caramels 1–4 denote increasing degrees of caramelization). Inset: representative images of ethylene glycol and α-bromonaphthalene droplets on the measured surfaces. Error bars indicate standard deviations of surface energies for different surfaces. (b) Glass-transition temperatures (Tg) of caramel samples prepared at four degrees of caramelization. (c) Schematic comparison of PMMA-assisted and caramel-assisted transfer. A relatively rigid PMMA support may not fully conform to the topography of the target substrate, potentially resulting in wrinkles or tearing during support removal. By contrast, the caramel support becomes compliant upon mild heating and application of gentle pressure, enabling it to conform to the target surface. (d) DFT-computed adsorption energies of representative caramelization products on graphene. (e) Photographs demonstrating transfer of monolayer graphene onto a range of substrates.

图4、Surface morphology of caramel- and PMMA-transferred graphene. (a, d) OM, (b, e) SEM and (c, f) AFM images of (d–f) PMMA- and (a–c) caramel-transferred graphene films.

图5、Chemical cleanliness, structural integrity, and surface residues of caramel- and PMMA-transferred graphene. (a–d) High-resolution C 1s XPS spectra and peak deconvolution of (a) pristine graphene/Cu (G/Cu), (b) alternating-field-treated graphene/Cu, (c) graphene/SiO2/Si transferred using caramel, and (d) graphene/SiO2/Si transferred using PMMA. The additional oxygenated-carbon features in (d) are consistent with PMMA-derived residues. (e) Raman spectra of as-grown G/Cu, oxidized G/Cu, caramel-transferred graphene, and PMMA-transferred graphene. (f) Correlation between the G- and 2D-band positions of caramel- and PMMA-transferred graphene. The blue dashed line with a slope of approximately 2.2 indicates the characteristic strain-related shift direction, whereas the red dashed line with a slope of approximately 0.7 indicates the p-type (hole) doping-related shift direction. The star marks the experimental reference position (1586, 2693 cm−1) obtained from as-grown G/Cu under the same Raman acquisition conditions. (g) Distributions of the 2D-band linewidth (Γ2D) for caramel- and PMMA-transferred graphene; the solid curves represent fits to the corresponding distributions. (h, i) Raman maps of Γ2D for PMMA- and caramel-transferred graphene, respectively. (j–l) Representative ToF-SIMS ion maps showing (j) a PMMA-related fragment and (k) Cu on PMMA-transferred graphene, and (l) a caramel-related oxygen-containing fragment on caramel-transferred graphene.+

图6、Electrical transport characteristics of caramel- and PMMA-transferred graphene devices. (a) Schematic of a Hall-bar device. (b) Photograph of fabricated Hall-bar arrays; each 4-inch Si wafer contains 20 device arrays, each array comprising 25 Hall bars (500 devices in total). (c) Optical micrograph of representative Hall-bar devices. (d) Output characteristic and (e) transfer characteristic of a representative caramel-transferred device. (f) Statistical distribution of hole mobility for caramel- and PMMA-transferred devices. (g) Output characteristic and (h) transfer characteristic of a representative PMMA-transferred device. (i) Statistical distribution of electron mobility for caramel-transferred devices.

本研究将界面氧化剥离与水溶性焦糖支撑结合，提出了一种大面积石墨烯清洁、低缺陷转移的可扩展路线。结果表明，在 50 Hz、名义场强 300 V/mm 交变电场中处理 5 h，石墨烯/铜界面氧化 1 h 即达约 70%、约 3 h 超过 90%（纯水对照 12 h 仅约 60%），纹理对比度由 0.25 降至 0.14，界面形成富 Cu₂O 状态，利于低损伤剥离；焦糖支撑层（Tg 约 58–69 °C）与多种衬底共形贴合、转移后可水溶去除。利用该方法实现 4 英寸石墨烯转移，薄膜 RMS 粗糙度仅 0.73 nm（优于 PMMA 的 1.53 nm），2D 峰平均线宽约 25.5 cm⁻¹（窄于 PMMA 的 36.9 cm⁻¹），p 型掺杂更弱；对 25 个霍尔器件的统计显示焦糖转移石墨烯平均空穴迁移率约 3396.9 cm²/(V·s)、电子迁移率约 503.3 cm²/(V·s)，而 PMMA 对照空穴迁移率仅约 226.8 cm²/(V·s)，焦糖组电导约为其 2 倍。因此，该工作为大面积石墨烯的清洁、低损伤、规模化转移与器件集成提供了新思路。需要指出，焦糖组器件的迁移率离散性仍较大、电子与空穴迁移率不对称，石墨烯/二氧化硅界面散射问题尚未完全解决，后续可进一步均匀化界面状态并拓展至其他二维材料与异质结构。

文献：https://doi.org/10.1002/smtd.71146