Fig. 1 Schematic illustration of the deposition of ceramic thin film from aqueous solution on SAMs
Nanoscale science and technology rely on control of phenomena occurring at the molecular and meso-structural level. Recently, nanoparticles and nanowires were applied to biosensing and biolabeling. For stability and biocompatibility, gold has long been used in biological studies. Gold nanoparticles are now being developed for labeling, DNA/drug delivery, and gene regulation. Most of these methods rely on modifying the surface with a specific molecule to which the molecule of interest is covalently bonded. One of the limitations of this approach is that once a molecule is bound to the carrier particle, the strong covalent bond prohibits release of the molecule into solution.
Recently, a technique applicable to research on low-temperature deposition of ceramic thin films has been developed, which is based on the use of organic self-assembled monolayers (SAMs, Figure 1) to promote film deposition. With different surface functional groups, different types of SAMs are found to either promote or hinder the deposition of different ceramic thin films based on electrostatic interactions. Because bio-materials, such as DNA, peptides, or proteins also hydrolyze in aqueous solution in a similar manner to inorganic materials, the fundamental understanding gathered by studying the interaction between inorganic materials and SAMs could be applied to nano-biotechnology. In other words, bio-materials could be selectively adsorbed and desorbed by SAMs through electrostatic interactions.
Fig. 1 Schematic illustration of the deposition of ceramic thin film from aqueous solution on SAMs
In order to selectively immobilize molecules on SAMs, the surface properties need to be tailored to match the properties of the materials. For example, by tailoring the surface properties of organic SAMs such that the attraction interaction occurs, bio-materials can be adsorbed selectively on SAMs-bearing nano-particles. By changing the environmental pH across the isoelectric point (IEP) of either the adsorbent or the SAM, the electrostatic interaction will be inverted, and the molecule will be repelled and released into the environment. For biological applications, the available pH range is often limited and requires the carrier to have a specific IEP so that the surface potential inverts in the biological pH range. In other words, carriers with a specific IEP are required. For a given organic functional group, however, the surface properties are controlled mainly by the nature of the group. Therefore, it will be difficult to fine-tune the properties simply by using different functional groups. One of the possibilities to tailor the properties of a substrate is to prepare a SAM that has multiple functional groups. The resultant chemical composition of the surface can be determined with x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS).
The interfacial charge density at solid-liquid interfaces is a controlling factor in a number of phenomena, such as adhesion, wetting, ion adsorption, and biocompatibility. The zeta potential, which is the potential at the shear plane between the compact layer and the diffuse layer, is an important indicator of the surface charge. This property is a useful parameter for determining the electrokinetic chemical properties of both pristine and modified surfaces. For particles smaller than tens of micrometers, electrophoresis light scattering (ELS) or phase analysis light scattering (PALS) methods are used in this group. For larger particles or bulk substrates, streaming current is measured by pumping electrolytes through a micro-channel formed by the sample. Using carboxylic acid and amine bearing SAMs on a flat Au surface it is demonstrated that arbitrary IEP values between the extremes defined by exclusive use of an amine or a carboxylic acid could be achieved (Figure 2). Similarly, using amine and thiol bearing SAMs on Si surface, a range of IEP values can be obtained.
Fig. 2 Zeta-potential of SAMs with mixed functional group as a function of environmental pH.
One of the key factors in the field of nano-technology is the microcharacterization of materials and the understanding of interactions at atom/molecular level. These subjects (surface analysis techniques, electron microscopes, etc.) are used extensively to understand and improve the synthesis and processing of materials.
Based on surface analysis techniques like XPS and SIMS that are regularly used to characterize the chemical composition of organic SAMs discussed in the previous section, we are extending these analytical methods to study complicated organic structures. The nanostructure of a material significantly affects the properties of the resultant opto-electronic devices. Unlike inorganic semiconductors, which are usually crystalline, organic materials are mostly amorphous. As a result, it has been difficult to analyze the nanostructure inside organic electronics using well-established analytical techniques. Furthermore, the segregated amorphous and oriented phases of polymers used in organic electronics is highly sensitive to the fabrication process. Additional phase separation in polymer films is often found upon mixing with small molecules that have significantly different chemical structures. This phase separation is crucial to device efficiency. Therefore, investigating and understanding the relationship between fabrication parameters, nanostructures in the polymer film and device performance is valuable to prepare highly efficient, long lifetime organic electronics.
In responding to the need of profiling multi-layered organic thin-film, we recently developed a series of novel analytical methods that allow for the analysis of the interior structure of organic materials. For example, the vertical nanostructure of organic opto-electronic devices has been studied with XPS (Figure 3 and 4). Although cluster ion sputtering significantly altered the outer-most surface of inorganic materials, it caused insignificant damage to the organic surface. This difference from the generally used atomic ion-beams is because of the shallower damage range and enhanced sputtering rate of the cluster ion. Using this novel analytical technique we reported the observation of electron-migration of small molecules inside organic light emitting diodes (OLEDs).
Fig. 3 Depth profile of organic light-emitting diode device.
Fig. 4 Depth profile of inverted polymeric solar-cell.
By combining the high depth-resolution of cluster ion slicing and high lateral-resolution of scanning probe microscope, 3D molecule distribution of bulk heterojunction is observed (Figure 5). Since SEPM is used to generate the contrast of the difference in contact potential, the resulting 3D volume image contains 4th physical dimension in work function.
Fig. 5 3D volume image of a bulk heterojunction. (click the image for full resolution, ~50MB each)
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Anton Paar SurPASS Electro-Kinetic Analyzer (EKA) The instrument is used for measuring the zeta-potential of bulk materials. This zeta-potential is important to predict the stability if colloidal suspensions and the tendency to agglomerate. |
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Brookhaven 90Plus/ZetaPALs Dynamic Light Scattering (DLS) Based on the dynamic light scattering (DLS), this instrument can be used to determine the particle size and its zeta-potential. Comparing with the electro-kinetic analyzer (EKA) like the SurPASS, this instrument is mainly for nano materials. Comparing with observing the particle size directly with electron microscopes, DLS is quick and non-destructive. |
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PHI VersaProbe XPS Microprobe X-ray Photoelectron Spectrometry (XPS), a.k.a. Electron Spectroscopy for Chemical Analysis (ESCA) In a ultra-high vacuum (UHV) chamber (1E-10 torr), photoelectrons are excited with X-ray. As the escape depth is shallow, the information came from the top few nm. The is important in studying the out-most chemical composition and chemical structure of materials. Combine the the sputtering gun (Ar and/or C60), one can slowly remove the surface and profile the depth. In combination with a quadrupole mass analyzer, this system also serves as a secondary ion mass spectrometer (SIMS). Operation Notes Training Material of XPS Training Video of XPS [part 1] [part 2] [part 3] [part 4] Training Material of SIMS Training Video of SIMS [part 1] [part 2] [part 3] [part 4] |
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FEI Nova200 NanoSEM Scanning Electron Microscope (SEM) Using a electron beam, surface structures can be observed directly with nm spatial resolution. With the high-resolution low-vacuum mode, non-conducting samples can also be observed with ~nm resolution. Combine with the X-ray Energy Dispersive Spectroscopy (XEDS), the chemical composition can also be determined. Training Material of SEM (part I) Training Material of SEM (part II) Training Material of EPMA Operation Notes |
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Veeco Innova SPM Scanning Probe Microscope (SPM) Using a varity of probes, a wide range of surface chemical, physical, mechanical, and electrical properties can be studied with high spatial resolution ( The system is sitting on a Halcyoncs Micro 40 active vibration isolation platform. The vibration level on the surface is better than 5 dB at <10 Hz and <0 dB for higher frequencies. |
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Q-SENSE E4 Qrartz Crystal Microbalance By measuing the change in freqiency of a quartz crystal resonator, mass change per unit area can be measured down to 1 ng/Hz-cm2. In addition to frequency, energy dissipation can also be measured to study the rigidity of deposited film. By using high-order overtones, the system is more stable in liquid environments and provide viscoelastic properties of the film. |
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JEOL JEM-2100F Transmission Electron Microscope (TEM), managed by the Core Facilities for Nanoscience and Nanotechnology The high-resolution electron microscope (HREM) has a resolution about angstrom. Atomic arrangements can be observed directly with this instrument. |
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High-performance parallel computing cluster This is for high-performance computing. The main system consists with 48 computing nodes. Each node has two dual-core 3GHz Woodcrest CPU and 8-32Gb fully-buffered memory. |