1成果簡介

柔性光電探測器因在可穿戴健康監(jiān)測、機(jī)器人視覺、柔性顯示等領(lǐng)域的廣闊前景，受到學(xué)術(shù)界和產(chǎn)業(yè)界的高度關(guān)注。鍺（Ge）作為IV族窄帶隙半導(dǎo)體，在可見-近紅外波段具有優(yōu)異的光吸收能力，是構(gòu)建寬帶柔性光電探測器的理想候選材料。然而，超薄柔性鍺薄膜中界面調(diào)制與光電響應(yīng)的物理機(jī)制尚不清晰，且柔性器件在持續(xù)變形條件下如何保持結(jié)構(gòu)完整性并實(shí)現(xiàn)可調(diào)控的光電響應(yīng)仍是重大挑戰(zhàn)。

本文，寧波大學(xué)Gang Wang、山東大學(xué)郭慶磊教授，中國科學(xué)院上海微系統(tǒng)與信息技術(shù)研究所薛忠研究員等在《Nano Letters》期刊發(fā)表題為"Multiphysics-Coupled Strain Engineering in Flexible 3D-Graphene/Germanium Heterostructures for Broadband Photodetection"的研究論文。研究在4英寸柔性Ge薄膜（~15 μm厚）上成功構(gòu)建了3D-石墨烯/鍺（3D-Graphene/Ge）異質(zhì)結(jié)，提出了一種多物理場耦合應(yīng)變工程策略，系統(tǒng)揭示了超薄柔性鍺中界面調(diào)制與光電響應(yīng)的物理機(jī)制。該柔性光電探測器展現(xiàn)出以下優(yōu)異性能：

• 超寬帶響應(yīng)380–1850 nm（紫外-可見-近紅外全覆蓋）
• 高響應(yīng)度80.2 A W?1（@1850 nm）
• 高探測率1.2 × 1011 Jones（@1850 nm）
• 超低噪聲2.2 × 10?21 A2 Hz?1
• ??快速響應(yīng)：上升/下降時(shí)間僅164/161 μs
• -3 dB截止頻率1.8 kHz
• 彎曲穩(wěn)定性>10?次彎曲循環(huán)+動(dòng)態(tài)變形后性能穩(wěn)定
• 
  

研究還將器件成功應(yīng)用于多光譜成像和可穿戴光電容積脈搏波（PPG）監(jiān)測（指尖和腕部），展示了其在柔性電子和生物醫(yī)學(xué)領(lǐng)域的實(shí)用化潛力。

2圖文導(dǎo)讀

圖1. Fabrication, morphological characterization, and built-in electric field properties of the 3D-graphene/Ge heterojunction. (a) A photograph of a 4-in. wafer-scale flexible Ge film. (b) A digital photograph demonstrating the bendability of the flexible Ge film; the inset presents a cross-sectional SEM image of Ge. (c) A cross-sectional SEM image of the 3D-graphene/Ge heterostructure. (d) Three-dimensional AFM image of the 3D-graphene/Ge heterostructure. (e) Porosity map of 3D-graphene. (f) Raman spectra of the 3D-graphene and Ge before and after integration; the inset shows the contact angle measurement. (g) Surface potential mapping of the flexible 3D-graphene/Ge heterojunction under dark and illuminated conditions. (h) Corresponding surface potential variation profiles. (i) Surface current mapping of the flexible heterojunction under dark and illuminated conditions. (j) Corresponding surface current variation profiles.

圖2. Photoelectric performance of the 3D-graphene/Ge heterojunction photodetector. (a) I–V characteristics of the flexible 3D-graphene/Ge heterojunction photodetector measured at different wavelengths. (b) Frequency-dependent noise current spectrum of the device. (c) Photocurrent response of the device to cyclic variations in incident laser power. (d) Photoresponse of the device under illumination with an 1850 nm laser at different modulation frequencies. (e) Attenuation characteristics of the photocurrent amplitude as a function of modulation frequency. (f) tr and tf of the device. (g) Self-powered photoresponse stability of the device; the inset shows the photocurrent variation over 100 consecutive cycles. (h) Responsivity and specific detectivity of the device at different wavelengths.

圖3. Regulation effects and physical mechanisms of mechanical deformation on the photoelectric performance of the flexible 3D-graphene/Ge heterojunction. (a) A schematic illustration of the 3D-graphene/Ge flexible heterojunction in a stable concave bending state. (b) TCAD-simulated distribution of photogenerated carriers in the flexible heterojunction under concave bending with optical illumination. (c) FDTD-simulated stress distribution and (d) electric field mode distribution of the flexible heterojunction under concave bending at different deformation levels. (e) A schematic diagram of the bending test setup for the flexible heterojunction, indicating the bending radius (r) and bending strain (ε). (f) I–V characteristics of the flexible heterojunction under different strain levels with 1850 nm illumination. (g) The dependence of the responsivity of the flexible heterojunction on bending strain. (h) A macroscopic optical photograph of bare flexible Ge under external bending. (i) Stability of the photoresponse signal of the flexible heterojunction during cyclic bending tests.

圖4. Applications of the 3D-graphene/Ge heterojunction in image sensing and flexible physiological signal monitoring. (a) A schematic illustration of the image sensing test platform. (b) A flowchart of signal processing and feature extraction for multiwavelength photoelectric responses. (c) A schematic diagram of pixel partitioning of the raw data. (d) The mask pattern used for imaging tests (size: 7 mm × 7 mm). (e–h) Photocurrent distribution images acquired by the photodetector under illumination at 780, 980, 1550, and 1850 nm, respectively. (i) The fused and visualized result of multiwavelength image integration. (j) A schematic of the fingertip-wearable testing setup, along with the corresponding heart rate measurement results under (k) resting and (l) postexercise conditions. (m) A schematic illustration of the wrist-based detection configuration, along with the corresponding heart rate measurement results under (n) resting and (o) postexercise conditions.

3小結(jié)

在本研究中，通過將晶圓級(jí)（4英寸）柔性鍺薄膜（約15微米）與原位生長的三維互連石墨烯相結(jié)合，構(gòu)建了一種柔性三維石墨烯/鍺異質(zhì)結(jié)構(gòu)。這種結(jié)構(gòu)制備出了性能優(yōu)異、光譜響應(yīng)寬廣且在機(jī)械變形下具有強(qiáng)長期穩(wěn)定性的柔性光探測器。實(shí)驗(yàn)與模擬結(jié)果表明，當(dāng)垂直取向的3D石墨烯多孔網(wǎng)絡(luò)與柔性鍺基板緊密耦合時(shí)，可增強(qiáng)光子吸收和載流子傳輸。所制備的柔性光探測器在380至1850 nm波長范圍內(nèi)表現(xiàn)出穩(wěn)定的光響應(yīng)，在1850 nm處的響應(yīng)度為80.2 A W –1，比探測率達(dá)1.2 × 1011瓊斯，低噪聲電流密度為2.2 × 10?21 A2 Hz?1，響應(yīng)速度達(dá)微秒級(jí)（tr和tf分別為164和161 μs），且-3 dB截止頻率為1.8 kHz。該研究揭示了柔性異質(zhì)結(jié)構(gòu)中機(jī)械變形、應(yīng)變梯度與電磁場之間的耦合機(jī)制。彎曲引起的應(yīng)變通過石墨烯的三維結(jié)構(gòu)增強(qiáng)了界面電勢和電場分布，從而產(chǎn)生應(yīng)變誘導(dǎo)的內(nèi)建電場。這種增強(qiáng)效應(yīng)改善了光生載流子的分離，并減少了非輻射復(fù)合損耗，確保了在彎曲和加載循環(huán)過程中光電性能的穩(wěn)定性。該器件在經(jīng)歷反復(fù)彎曲循環(huán)和長期儲(chǔ)存后，性能退化可忽略不計(jì)，彰顯了其機(jī)械可靠性和工作穩(wěn)定性。這種柔性三維石墨烯/鍺光探測器已成功應(yīng)用于多波長高分辨率成像和精確生理信號(hào)采集，實(shí)現(xiàn)了從可見光到近紅外光譜范圍內(nèi)的有效波長分辨、空間定位以及持續(xù)的心率動(dòng)態(tài)監(jiān)測。該研究成果證實(shí)了該技術(shù)在柔性光電子器件領(lǐng)域的可行性和可靠性，對(duì)可穿戴光電子設(shè)備、多光譜成像、柔性生物電子學(xué)及先進(jìn)傳感平臺(tái)具有重要意義。

文獻(xiàn)：
https://doi.org/10.1021/acs.nanolett.6c00834

來源：材料分析與應(yīng)用