使用者:Schenad/朗繆爾-布洛傑特薄膜

布儒斯特角顯微鏡下浮於水面上的，由磷脂構成的朗繆爾薄膜。

朗繆爾-布洛傑特薄膜（Langmuir–Blodgett film），或LB薄膜、LB膜，得名於歐文·朗繆爾和凱瑟琳·B·布洛傑特，是一類由人工製備的單分子層。朗繆爾-布洛傑特薄膜的製備需先在某液體表面形成單分子層；將一固體從單分子層區域緩慢浸入液體（或從液體中抽出）就可以讓單分子層轉移到固體表面。通過這一方式附着在固體表面的這一單分子層即為朗繆爾-布洛傑特薄膜。需要注意的是，朗繆爾-布洛傑特薄膜特指已被轉移到固體表面的單分子層；而朗繆爾薄膜（Langmuir film）指的是漂浮於液體表面的單分子層。通過反覆浸入與抽出的操作，液體表面的朗繆爾薄膜可以被一層層地轉移到固體表面，而不同的操作次數將製備出不同厚度的單分子層。單分子層既可以是兩親分子（包含親水的頭部和疏水的尾部，例如脂肪酸），也可以由納米顆粒構成。

歷史背景

早在朗繆爾和布洛傑特的發現之前，本傑明·富蘭克林就已於1773年完成了相關的實驗。當時，富蘭克林將大約一湯匙的油倒入一個小池塘之後，約有半英畝水面的漣漪在他倒入油的一瞬間消失了。然而，富蘭克林並沒有意識到水面上的油膜是單分子層。一個多世紀之後，約翰·斯特拉特，第三代瑞利男爵根據富蘭克林的實驗計算出當時形成的油膜的厚度約為1.6納米。

阿尼亞斯·泡克耳斯通過在自家廚房水槽中做的一系列實驗展現了如何用屏障控制薄膜的面積。She added that surface tension varies with contamination of water. She used different oils to deduce that surface pressure would not change until area was confined to about 0.2 nm². This work was originally written as a letter to Lord Rayleigh who then helped Agnes Pockels become published in the journal, Nature, in 1891.

Sarfus image of one Langmuir–Blodgett monolayer of stearic acid (thickness=2.4nm).

Agnes Pockels』 work set the stage for Irving Langmuir who continued to work and confirmed Pockels』 results. Using Pockels』 idea, he developed the Langmuir (or Langmuir–Blodgett) trough. His observations indicated that chain length did not impact the affected area since the organic molecules were arranged vertically.

Langmuir’s breakthrough did not occur until he hired Katherine Blodgett as his assistant. Blodgett initially went to seek for a job at General Electric (GE) with Langmuir during her Christmas break of her senior year at Bryn Mawr College, where she received a BA in Physics. Langmuir advised to Blodgett that she should continue her education before working for him. She thereafter attended University of Chicago for her MA in Chemistry. Upon her completion of her Master's, Langmuir hired her as his assistant. However, breakthroughs in surface chemistry happened after she received her PhD degree in 1926 from Cambridge University.

While working for GE, Langmuir and Blodgett discovered that when a solid surface is inserted into an aqueous solution containing organic moieties, the organic molecules will deposit a monolayer homogeneously over the surface. This is the Langmuir–Blodgett film deposition process. Through this work in surface chemistry and with the help of Blodgett, Langmuir was awarded the Nobel Prize in 1932. In addition, Blodgett used Langmuir–Blodgett film to create 99% transparent anti-reflective glass by coating glass with fluorinated organic compounds, forming a simple anti-reflective coating.

物理原理

LB films are formed when amphiphilic molecules like surfactants or nanoparticles interact with air at an air–water interface. Surfactants (or surface-acting agents) are molecules with hydrophobic 'tails' and hydrophilic 'heads'. When surfactant concentration is less than critical micellar concentration (CMC), the surfactant molecules arrange themselves as shown in Figure 1 below. This tendency can be explained by surface-energy considerations. Since the tails are hydrophobic, their exposure to air is favoured over that to water. Similarly, since the heads are hydrophilic, the head–water interaction is more favourable than air–water interaction. The overall effect is reduction in the surface energy (or equivalently, surface tension of water).

Figure 1: Surfactant molecules arranged on an air–water interface

For very small concentrations, far less than critical micellar concentration (CMC), the surfactant molecules execute a random motion on the water–air interface. This motion can be thought to be similar to the motion of ideal-gas molecules enclosed in a container. The corresponding thermodynamic variables for the surfactant system are, surface pressure ($\Pi$), surface area (A) and number of surfactant molecules (N). This system behaves similar to a gas in a container. The density of surfactant molecules as well as the surface pressure increases upon reducing the surface area A ('compression' of the 'gas'). Further compression of the surfactant molecules on the surface shows behavior similar to phase transitions. The 『gas』 gets compressed into 『liquid』 and ultimately into a perfectly closed packed array of the surfactant molecules on the surface corresponding to a 『solid』 state.

The condensed films is subsequently deposited on a solid substrate to create highly organized thin film coatings. Langmuir–Blodgett troughs are used for this purpose.

Besides LB film from surfactants depicted in Figure 1, similar monolayers can also be made from inorganic nanoparticles.

Pressure–area characteristics

Adding a monolayer to the surface reduces the surface tension, and the surface pressure, $\Pi$ is given by the following equation:

$\Pi =\gamma _{0}-\gamma$

where, $\gamma _{0}$ is equal to the surface tension of the water and $\gamma$ is the surface tension due to the monolayer. But the concentration-dependence of surface tension (similar to Langmuir isotherm) is as follows:

$\gamma _{0}-\gamma$ = RTK_HC = – RT$\Gamma$

Thus,

$\Pi =RT\Gamma$ or,
$\Pi A=RT.$

The last equation indicates a relationship similar to ideal gas law. However, it should be noted that the concentration-dependence of surface tension is valid only when the solutions are dilute and concentrations are low. Hence, at very low concentrations of the surfactant, the molecules behave like ideal gas molecules.

Experimentally, the surface pressure is usually measured using the Wilhelmy plate. A pressure sensor/electrobalance arrangement detects the pressure exerted by the monolayer. Also monitored is the area to the side of the barrier which the monolayer resides.

Figure 2. A Wilhelmy plate

A simple force balance on the plate leads to the following equation for the surface pressure:

$\Pi =-\Delta \gamma =-\left[{\frac {\Delta F}{2(t_{p}+w_{p})}}\right]\approx -{\frac {\Delta F}{2w_{p}}}$,

only when $w_{p}\gg t_{p}$.

Here, $l_{p},w_{p}$ and $t_{p}$ are the dimensions of the plate, and $\Delta F$ is the difference in forces. The Wilhelmy plate measurements give pressure – area isotherms that show phase transition-like behaviour of the LB films, as mentioned before (see figure below). In the gaseous phase, there is minimal pressure increase for a decrease in area. This continues until the first transition occurs and there is a proportional increase in pressure with decreasing area. Moving into the solid region is accompanied by another sharp transition to a more severe area dependent pressure. This trend continues up to a point where the molecules are relatively close packed and have very little room to move. Applying an increasing pressure at this point causes the monolayer to become unstable and destroy the monolayer.

Figure 3. (i) Surface pressure – Area isotherms. (ii) Molecular configuration in the three regions marked in the $\Pi$-A curve; (a) gaseous phase, (b) liquid-expanded phase, and (c) condensed phase. (Adapted from Osvaldo N. Oliveira Jr., Brazilian Journal of Physics, vol. 22, no. 2, June 1992)

研究應用

• 朗繆爾-布洛傑特圖案生成（Langmuir–Blodgett patterning）是一種用於大面積的、介孔結構圖案生成（large-area patterning）的新範式。
• 最近，在製備大面積的二維層狀材料的超薄膜時，人們發現朗繆爾-布洛傑特方法不失為一種有效的方式。

另見

• 布儒斯特角顯微鏡
• 朗繆爾-布洛傑特槽
• 納米粒子沉積
• 自組裝單分子層
• 威氏平板

參考書目

• R. W. Corkery, Langmuir, 1997, 13 (14), 3591–3594
• Osvaldo N. Oliveira Jr., Brazilian Journal of Physics, vol. 22, no. 2, June 1992
• Roberts G G, Pande K P and Barlow, Phys. Technol., Vol. 12, 1981
• Singhal, Rahul. Poly-3-Hexyl Thiopene Langmuir-Blodgett Films for Application to Glucose Biosensor. National Physics Laboratory: Biotechnology and Bioengineering, p 277-282, February 5, 2004. John and Wiley Sons Inc.
• Guo, Yinzhong. Preparation of poly(N-alkylmethacrylamide) Langmuir–Blodgett films for the application to a novel dry-developed positive deep UV resist. Macromolecules, p1115-1118, February 23, 1999. ACS
• Franklin, Benjamin, Of the stilling of Waves by means of Oil. Letter to William Brownrigg and the Reverend Mr. Farish. London, November 7, 1773.
• Pockels, A., Surface Tension, Nature, 1891, 43, 437.
• Blodgett, Katherine B., Use of Interface to Extinguish Reflection of Light from Glass. Physical Review, 1939, 55,
• A. Ulman, An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly, Academic Press, Inc.: San Diego (1991).
• I.R. Peterson, "Langmuir Blodgett Films ", J. Phys. D 23, 4, (1990) 379–95.
• I.R. Peterson, "Langmuir Monolayers", in T.H. Richardson, Ed., Functional Organic and Polymeric Materials Wiley: NY (2000).
• L.S. Miller, D.E. Hookes, P.J. Travers and A.P. Murphy, "A New Type of Langmuir-Blodgett Trough", J. Phys. E 21 (1988) 163–167.
• I.R.Peterson, J.D.Earls. I.R.Girling and G.J.Russell, "Disclinations and Annealing in Fatty-Acid Monolayers", Mol. Cryst. Liq. Cryst. 147 (1987) 141–147.
• A.M.Bibo, C.M.Knobler and I.R.Peterson, "A Monolayer Phase Miscibility Comparison of the Long Chain Fatty Acids and Their Ethyl Esters", J. Phys. Chem. 95 (1991) 5591–5599.

[[Category:纳米技术]] [[Category:物质状态]] [[Category:薄膜]]