利用明膠與石墨烯發展非揮發性水相記憶體

Abstract

目前許多研究致力於發展可撓式、耐用、且可以在有水的環境下操作的電子產品，以利於偵測環境變化以及人體健康。然而水的存在對於記憶體裝置有極大的影響，像是傳統的電阻式記憶體或是利用浮動閘極場效電晶體做成的記憶體裝置，很容易受潮濕的環境或是電解質水溶液的存在影響，而流失其存取的資訊。在本論文中，我們利用具有生物相容性的材料來發展水相記憶體裝置，期望未來能應用在穿戴式裝置或是機器及人體間溝通的介面。 首先，我們發現蛋白質明膠可以透過氫鍵形成的網絡來傳遞電子，且傳遞的距離可以達到微米等級。一般飽含水分的明膠只能做為電容，然而當被加熱至融化並且散失水分之後，所形成的明膠-氫鍵網絡則可以導電。由於擁有從電容轉變為高導電度電阻的特性，我們利用明膠來發展非揮發性電阻式記憶體，且其開關電流比值在0.09伏特的讀取電壓下可以高達105；相較於目前擁有的非揮發性電阻式記憶體，此裝置可以在至少小十倍以下的讀取電壓達到高開關電流比值。此外，此明膠記憶體具有高達5天的穩定性，也可以透過恢復其水分含量來刪除已寫入資訊。 接著，我們更進一步發現在明膠中加入視紫蛋白除了可以增加其導電度，也可以提升其記憶體表現。視紫蛋白為光驅動氫離子幫浦，在吸收光能之下可以製造細胞膜兩側的氫離子濃度差異。我們的實驗結果顯示在明膠記憶體中間加入一層具有視紫蛋白的脂質膜，會顯著加強明膠-氫鍵網絡傳遞電子的能力。我們認為視紫蛋白在光驅動後，會讓脂質膜一側的明膠多出氫離子，另一側則是少了氫離子。而我們認為這樣的情況會造成明膠-氫鍵網絡的結構變化，進而增加其電子傳遞的能力。此外，加入視紫蛋白的明膠記憶體也一樣用有長期的穩定性及重新編輯的功能，而且其開關電流比值比單純利用明膠做成的記憶體高了3倍。利用明膠來做為記憶體的材料，讓我們可以透過控制溫度及水分含量來進行寫入、讀取、刪除資訊，且可以透過加入視紫蛋白來提升其表現，相信未來可以進一步被應用於生物相關之電子產品領域。 除了明膠之外，我們也利用石墨烯電晶體來發展水相記憶體。我們發現在石墨烯和玻璃間形成的奈米厚度之水層會大幅影響石墨烯的導電特性；此外，在寫入及刪除的過程中，我們可以透過施加正負不同的閘極電壓來控制此水層的形成，並達到穩定的高導電度態(開)以及低導電度態(關)。此石墨烯記憶體可以進行至少9次的重複寫入-讀取-刪除，且可以維持在開與關的狀態至少104秒。我們也進一步利用原子力顯微鏡和拉曼光譜來檢驗此兩狀態下水層厚度的差異。此利用奈米水層來調控石墨烯與玻璃間相互作用的技術，未來可應用於發展在水中操做之極薄二維型態的非揮發性記憶體。

The development of flexible, robust electronics functional in aqueous systems have attracted great attention owing to their potential abilities for environmental monitoring and health sensing. However, the memory devices are highly sensitive to the existence of water. The stored electrical information can be easily lost when a conventional resistive memory operate in an electrolyte solution or in a wet environment. In this thesis, we devoted to develop water-based memory devices using biocompatible materials for the future applications in wearable devices and human-machine interfaces. First, we discovered that a natural protein, gelatin, can form a gelatin-hydrogen-bond network that can transfer electrons over a long range (at least hundreds of micron). A typical gelatin hydrogel rich in water can only act as a capacitor with no electron transfer across the gel. We found that an electron-conductive gelatin-hydrogen-bond network can be formed if the water content of the typical wet hydrogel is reduced above its gelling temperature. The switch from a capacitor with low conductivity to a resistor with high conductivity allows us to use a gelatin thin film as a resistor-type memory device. The gelatin thin film exhibits nonvolatile resistive memory features with a high ON/OFF current ratio on the order of 105 at a reading voltage of 0.09V. The high ON/OFF ratio is achieved at the low reading voltage which is at least one order of magnitude lower than the one required by current nonvolatile resistive memory devices. In addition, it has a long-term stability up to 5 days, and the ability to be reprogrammed by water addition. Second, we discovered that incorporating a lipid membrane rich in a light-driven proton pump, bacteriorhodopsin (BR), into the gelatin device could further increase the conductivity and improve the memory performance. After the light illumination, BR can create additional protons in the gelatin on one side of the membrane and proton vacancies in the gelatin on the other side of the membrane. We think the proton accumulation or proton deficiency may affect the structure of the gelatin-hydrogen-bond network and therefore enhance the electron transport in the gelatin device. The BR-incorporated gelatin memory device also features long-term stability and reprogrammability; furthermore, the ON/OFF current ratio is increased by 3 folds compared with the gelatin memory device with no BR-lipid-membrane. Using a gelatin thin film as a memory device provides a new pathway for writing, reading, and erasing by controlling the structure and water content in the gelatin, and the memory performance can be enhanced by incorporating a BR-lipid-membrane, which can be further used for various bioelectronic applications. Third, we present a solution-gated graphene memory device based on the interaction between graphene and its silica substrate. We discovered that the water layer thickness between the graphene and the silica substrate can significantly influence the electrical property of the graphene. More importantly, we can construct stable high-conductance state (ON state) and low-conductance state (OFF state) by applying positive and negative gate voltages to control the water layer thickness as the writing and erasing processes. We show that the device can undergo over at least 9 WRER cycles and the ON and OFF states can be maintained at least to 104 s. The AFM and Raman spectroscopy data of the ON and OFF states support the water layer thickness difference of the two states. That the graphene memory device can robustly function in water can make the device suitable for various applications. The manipulation of the interaction between a graphene sheet and silica through a thin water layer can also facilitate the realization of a water-based ultra-compact 2D nonvolatile memory.