奈米流體二極體與電晶體內之離子傳輸機制：鹽濃度梯度、溫度梯度與表面帶電之影響

Abstract

由於奈米製造技術的迅速發展，奈米尺度的孔道已經成功被製備並廣泛應用，許多學者正著手探討這類裝置內的電動力學輸送現象。在第一章中，我們考慮了一個在施加外加電場、鹽濃度梯度及溫度梯度下，具有雙極性聚電解質層的圓錐形奈米孔道。藉由數值模擬，我們發現系統中的電動力學行為主要由鹽濃度梯度決定，並受到溫度梯度之輔助。此外，孔道內之離子累積與消耗主要受到聚電解質層中帶電區域的屏蔽效應所影響，此可透過改變鹽濃度梯度和溫度梯度方向（共四種可能之組合）來實現。如果施加小的負電壓，奈米孔道會由陰離子選擇性轉變為陽離子選擇性，且當孔道尖端側的濃度高時，此現象尤其明顯；然而，該現象與溫度無關。當施加鹽濃度梯度時，離子電流整流比會在孔道尖端側的濃度高時出現最大值，而在另一側濃度高時呈現單調遞減。當施加溫度梯度，奈米孔道的整流能力僅會受些微影響。離子選擇性在正電壓下主要受溫度影響，而在負電壓下則由鹽濃度主導。以上研究成果已發表在《Electrochimica Acta》期刊。 在第二章中，基於納維-斯托克斯和泊松-奈恩斯特-普朗克方程的連續體模型，我們模擬出由兩種金屬氧化物（氧化鋁和二氧化鈦）所組成之奈米流體電晶體中的離子傳輸行為。透過改變電解質溶液的pH值，使其橫跨兩種材料的等電點，可進一步調節奈米孔道中三個區域的表面帶電型態，此現象是基於固液界面上帶電官能基團的解離變化。我們考慮了幾個關鍵因素，包含圓錐開口角度(θ)、閘極比例長度(Ω)、電解質溶液濃度(C_"KCl" )以及水解反應的發生。在五個特定的pH值之下，模擬結果皆顯示離子導電率(G)在θ變大時先增後減，而在增加C_"KCl" 時則單調遞增。在高pH之下，G會隨著Ω變大而持續增加；但在中及低pH之下，G則隨著Ω變大會先減後增。水解反應的影響在高pH之下最為劇烈，在正負帶電區域交界處的離子累積最為顯著。在高及中pH之下，奈米孔道的選擇性會有大幅變化，而在低pH之下僅有輕微改變。根據我們的研究結果可知，奈米流體電晶體的離子傳輸特性可以經由不同的孔道幾何形狀與外在環境的設計來有效獲得調控。

Owing to rapid development of nanofabrication technology, nanoscale channels have been successfully prepared and widely utilized. Electrokinetic transport phenomena within these devices are further investigated by researchers. In Chapter 1, we consider a conical nanopore functionalized with bipolar polyelectrolyte (PE) layers subject to applied electric field, concentration gradient (∇C), and thermal gradient (∇T). Through numerical simulation, we find that electrokinetic behavior of the system is primarily governed by ∇C, with the contribution of ∇T being auxiliary. In addition, ion accumulation and depletion within the pore are mainly influenced by the shielding effect of the charged region of PE layers through varying the directions of ∇C and ∇T (four possible combinations). If a small negative voltage bias is applied, an anion-selective nanopore switches to a cation-selective one, and this phenomenon is pronounced when a higher concentration is placed on the nanopore tip side. However, that phenomenon is temperature independent. When ∇C is applied, the ionic current rectification factor R_f shows a maximum value as a higher concentration is placed on the nanopore tip side, while it monotonically decreases as a higher concentration is placed on the other side. As for applying ∇T, the rectification capability of the nanopore is only slightly affected. The ion selectivity S mainly depends on temperature under a positive voltage bias; however, it becomes dominated by concentration when a negative voltage bias is applied. These research findings have been published in the journal Electrochimica Acta. In Chapter 2, a continuum model based on coupled Navier-Stokes and Poisson-Nernst-Planck (PNP) equations is conducted to simulate ion behaviors within a nanofluidic transistor consisting of two kinds of metal oxides, namely aluminum oxide (Al2O3) and titanium dioxide (TiO2). Through manipulating the pH level of the electrolyte solution across the isoelectric points (IEPs) of both constituent materials, active modulation of the surface charge pattern in each of the three segments of the nanopore is achievable, stemming from varying dissociation levels of charged functional groups at the solid-liquid interface. We introduce several crucial factors, including adjustments to geometric parameters such as the cone angle (θ) and proportional length of the gate (Ω), as well as modifications to the solution environment, encompassing variations in bulk concentration (C_"KCl" ) and occurrence of the water dissociation reaction. Considering five specific pH levels, the results demonstrate that ion conductance (G) experiences initial increases followed by declines with rising θ, and exhibits monotonic increases with increasing C_"KCl" , regardless of pH values. Continuous increases of G occur at high pH but transition to declines followed by increases at moderate and low pH with increasing Ω. Effects of the water dissociation reaction are most pronounced at high pH levels, where ion accumulation at the junction is notable. S of the nanopore undergoes profound alterations at high and moderate pH levels, while only minor changes are observed under low pH conditions. Drawing upon our findings, it becomes evident that the ion transport properties of a nanofluidic transistor can be effectively modulated through distinct geometric configurations and in response to diverse external conditions.