探討硒化鉛奈米顆粒陣列在不同耦合強度下的電性傳輸行為

Abstract

本實驗使用的硒化鉛奈米顆粒是以膠體合成(colloidal synthesis)方法製作，硒化鉛外圍會有一層有機包覆層，硒化鉛形狀為鈍角之正立方體，直徑約15 nm。藉由調控製程參數，以電子束微影技術以及熱蒸鍍技術製作間隙大約為100 nm之指狀電極，並以介電泳技術使硒化鉛奈米顆粒往電極間隙移動，並組裝成奈米顆粒陣列元件，本篇論文為探討硒化鉛奈米顆粒元件之電性行為。 奈米元件在真空中分別以100 ℃、200 ℃、225 ℃熱退火處理，並變溫量測其電性行為，退火溫度100 ℃與沒有熱退火處理的奈米元件，其室溫電阻值大於1 GΩ；退火溫度大於200 ℃，室溫電阻降至0.1 GΩ -5 MΩ，對應有機包覆層的裂解溫度為200 ℃，推測其電性變化為包覆層變薄、奈米顆粒間距變小造成電子偶合強度增加所致。在忽略充電能與考慮量子侷限能的情況下，我們認為低溫時為三維Mott變程跳躍傳輸機制(3-D mott variable range hopping)主導，而高溫時則為熱活化傳輸機制(thermally activated transport)與最近鄰跳躍傳輸機制(nearest-neighbour hopping)共同主導，最後以三維Mott變程跳躍機制、熱活化傳輸機制以及最近鄰跳躍傳輸機制並聯解釋整體傳輸行為。藉由分析溫度對電阻與電流對電壓關係，我們發現熱活化傳輸所需的活化能與變程跳躍傳輸中的跳躍能量在200 ℃及225 ℃熱退火處理後會有明顯的下降，其原因可能是因為奈米顆粒間距變小後，造成奈米顆粒間的耦合強度增強，使得電子的局域長度增長，當局域長度大於電極間距時，電子傳輸機制將由熱活化傳輸機制主導。

The electron transport in PbSe nanoparticle arrays at various strengths of inter-particle coupling is studied from temperature behaviors of current-voltage curves at a temperature range between 80 and 300 K. In this study, PbSe nanoparticles with an average diameter of 14.6 nm were attracted by dielectrophoresis and they form an array between two nano-scale electrodes patterned by electron beam lithography. The as-fabricated PbSe nanoparticle-array devices were highly insulating. In order to raise the inter-particle coupling, thermal annealing was employed to fine tune the inter-particle distance. Under different annealing conditions, we find that the resistances of the samples decrease with an increase of the annealing temperature. We argued that the charging energy and quantization effects are negligible small in our case so we used thermal activation of nearest neighbor hopping and Mott’s variable range hopping theories to explain our experimental results at high and low temperatures. Through scrutinizing all fitting parameters, we discovered a crossover from localized regime to extended regime which is possibly manipulated by the inter-particle coupling strength. In the localized regime, the activation energy at high temperature is used for an excitation from the Fermi level to the 1S state of PbSe nanopaticles for hopping transport between nanoparticles. In the extended regime, the activation energy of the sample at high temperature is half of the case in the localized regime, which evidences the absence of excitation energy.