The theme of this research group can be divided into two parts with a common aim of biological applications and support the progress of each other.
1) Surface and Interface Sciences: using self-assembled monolayers with mixed functional groups, interfacial properties can be tailored for studying the interaction between biological species and surfaces.
2) Analysis and Imaging of Soft Matters: based on surface analysis techniques that characterize surface and interface properties, a series of techniques are developed to analyze the structure inside soft matters directly.
Synthesis of 1D Materials
For ease of processing, anodic aluminum oxide (AAO) is used widely as a structure-directing template for preparing nanotubes. These processes, however, either require a vacuum apparatus or consist of filling the template with a pre-existing solid synthesized in separate steps. Another general approach to prepare nanowires is to electroplate materials inside the template after the evaporation of electrodes on the AAO template. To further simplify the preparation, a single-step direct deposition of oxides into the template under near-ambient conditions is required. Using AAO and carefully selected solution chemistry, precursor ions are adsorbed on the inner surface of the template. After in situ hydrolysis reactions, stoichiometric SrTiO3 and BaTiO3 nanowire and nanotubes are synthesized from aqueous solutions without pre-existing solids or an electric field (Inorganic Chemistry 2009, 48, 681-686).
Self-Assembled Monolayers with Mixed Functional Groups
Carboxylic acid- and amine-bearing SAMs hydrolyze to carboxylate anions and ammonium cations in an electrolyte solution and yield two distinct IEP values. Similarly, thiol group deportonated and posses a low IEP. By mixing these functional groups on a substrate, the opposite charges can cancel each other out, and arbitrary IEP values of flat Au (Physical Chemistry Chemical Physics 2009, 11, 6199-6204), Au nanoparticles (Journal of Colloid and Interface Science 2009, 340, 126-130) and Si surfaces (Physical Chemistry Chemical Physics 2011, 13, 3649-3653) between the extremes defined by individual functional groups could be achieved. These zeta-potentials are measured using independent techniques of streaming potential and dynamic light scattering and can be compared. In addition, the effect of the surface chemical composition of mixed functional group on the work function is also studied (Physical Chemistry Chemical Physics 2011, 13, 4335-4339; Physical Chemistry Chemical Physics 2011, 13, 15122-15126).
Zeta-potential of SAMs with mixed functional group as a function of environmental pH.
With this tunable system, the effect of the surface potential and the surface functional groups to the adsorption of materials are studied with a quartz crystal microbalance (QCM) with dissipation detection. The viscoelastic model is applied to determine the packing density and the thickness of the adsorbed films for studying the adsorption behavior of plasmid DNA on the surfaces of various potential. It is found that the adsorption behavior cannot be simply explained by the electrostatic interactions. Instead, because both DNA and SAM molecules are polarizable, induced dipole (Debye) interactions need to be taken into account as well. (Journal of Colloid and Interface Science 2012, 382, 97-104)
Ultimate film thicknesses on binary-SAMs of various ratios in different pH environments.
Degradation of Self-Assembled Monolayers on Au
The quality of SAMs is affected significantly by the deposition conditions. By simply adding HCl to the deposition solution, a high quality amine-SAM is obtained. Using various surface analytical techniques, including contact angle analysis, x-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS), the hydrophilic amino group was found to slowly oxidize to a hydrophobic nitroso group even under ambient conditions. As a consequence, the surface potential (and hence, the isoelectric point) of the SAM surface changed significantly. These changes were monitored with an electrokinetic analyzer (Journal of Physical Chemistry C 2010, 114, 10512-10519). These changes in surface properties can be suppressed by isolating the specimen from ambient light or air indicating the degradation is a photo-oxidation process.
Nanodot-Enhanced Organic Light Emitting Diodes (in collaboration with Prof. J.-H. Jou of NTHU)
Polysilicic acid nanodots were doped into the hole-transporting layer of an. Because of a more balanced carrier-injection, pure white light-emitting devices with up to a 3.5× higher efficiency were fabricated. To gain more knowledge of how the polysilicic nanodots (PNDs) affect the carrier transportation, the surface of the PNDs were modified with amino (Am-), vinyl (V-), and alkyl (Al-) moieties in addition to the intrinsic hydroxyl (H-) functional groups. The enhancement in device efficiency is controlled by the magnitude of surface charge and is independent to the sign of the charge and the bandgap of the light emitting dye (ACS Nano 2010, 4 4054-4060).
This novel analytical technique has also been used to study the degradation of an organic LED directly (Organic Electronics 2009, 10, 581-586; Organic Electronics 2011, 12, 376-382). Currently, there are no other means that can analyze the structure of a whole organic electronic operated for different times directly. In addition to the previously proposed degradation mechanisms, a new mechanism based on the direct observation of the migration of small molecules under direct current is reported. This mechanism has never been considered before and can explain the reported results. The knowledge gained in this work is crucial to the success of organic electronics with sufficient life-times for real applications.
3D Nanostructure inside Organic Electronic Devices
To gain high spatial resolution in 3D, the sputtering technique that provided excellent depth resolution was combined with scanning probe microscopy, which has high lateral resolution. By combining different instrumental techniques like scanning electrical potential microscopy (Analytical Chemistry 2009, 81, 8936-8941, highlighted by the journal, specimens are provided by Dr. C.-W. Chu) or force modulation microscopy (ACS Nano 2010, 4, 2547-2554), the nanostructure inside organic materials can be examined at high resolution. The fabrication parameters affect the resulting nanostructure significantly. For solar-cell devices, the phase-separation inside the bulk-heterojunction provides an adequate path for the removal of charge carriers that in turn enhance the device efficiency. On the other hand, phase-separation inside light-emitting devices is less efficient in trapping the charge carriers, which leads to efficiency drops with the development of a nanostructure.
3D volume image of a bulk heterojunction. (click the image for full resolution, ~50MB each)
In collaboration with ULVAC-PHI, a scanning ToF-SIMS with spatial resolution of 50 nm is also demonstrated (ACS Nano 2010, 4, 833-840; Analyst 2011, 136, 716-723). Using a bulk heterojunction solar cell (specimens are provided by Prof. J.-H. Jou) as an example, the 3D molecular distribution was obtained before and after annealing. This work revealed the important relationship between fabrication conditions, nanostructure, device architecture, and device performance.
Compared with established techniques like electron tomography, which can yield 3D nanostructures, the contrast generated by our technique is from direct physical properties instead of crystallinity that relates indirectly to the materials. Furthermore, ion sputtering has no physical limitations to the analyzable thickness. As a result, as long as the sample is stable to high-vacuum it can be studied in its original state without sample preparation that can introduce artifacts. Therefore our techniques are universal and more versatile than TEM-based techniques.
Electron Tomography of Biological Cells using SEM-Based STEM
In an apparatus based on SEM, which is relatively cheap and operates at a lower voltage (up to 30 kV), the electron interacts with the specimens more strongly and can generate better contrast, especially with light elements. Therefore, SEM-based electron tomography is developed By using a home-made sample holder that holds a standard 3-mm TEM grid in the chamber and allows a multi-segment solid-state electron detector to be inserted behind the specimen. The 3D volume structure is then reconstructed from 2D projections acquired from the STEM using the public domain TomoJ software. A thick (1 μm) specimen of an HEK293T cell could be imaged with sufficient information to examine Au nanoparticles in the cytosol using this modified instrument (Microscopy and Microanalysis 2012, 18, 1037-1042).
Integration of Dynamic-Secondary Ion Mass Spectrometer (D-SIMS) and X-ray Photoelectron Spectrometer (XPS)
Based on an existing XPS apparatus equipped with multiple ion guns, a quadrupole mass analyzer was integrated. Using the originally designed XPS function, the elemental and chemical state information of remaining surface after (cluster) ion sputtering can be acquired. Using the mass analyzer concurrently during sputtering, secondary ions emitted from the specimen can be acquired at the same time (Analyst 2011, 136, 941-946; Rapid Communications in Mass Spectrometry 2011, 25, 2897-2904; Analytical Chemistry 2012, 84, 9318-9323). Using this unique system, the role of auxiliary Ar+ in C60+-Ar+ co-sputtering is studied and the auxiliary Ar+ ion is suggested to be slowly removing the deposited C. Furthermore, the interaction between these ion beams in free space is confirmed and it is found that the excessive Ar+ fluence could break C60+ and slower the over-all sputter rate. On the other hand, if the fluence of auxiliary Ar+ is insufficient, it cannot remove the deposited C effectively hence yield unsteady sputtering process. As a result, the dosage of auxiliary ion beams has to be optimized.
The use of C60+ sputtering is further applied for for profiling biological materials. With the high ionization yield at high-mass regions, peptide molecular ions can be generated and detected directly, allowing for the parallel detection and quantification of multiple peptides without labeling. Compared to the established MALDI technique, co-ionization does not require a special matrix that contaminates the specimen, and the controlled ion-erosion process gives better depth resolution. Therefore, this co-sputtering technique is a leap forward for the parallel detection, quantification, and depth profiles of biological specimens using SIMS (Analytica Chimica Acta 2012, 718, 64-69). To further enhance the intensity of secondary ions hence increasing the sensitivity for biological species, it is found that C60+ sputtered surface oxidized quickly (Analytical Chemistry 2012, 84, 3355-3361) hence the neutralization of secondary ions is suppressed. Furthermore, by using O2+ as the auxiliary ion in the co-sputtering, molecular ion intensity increased significantly (Analytical Chemistry 2013, 85, 3781-3788). These results pave the way for successful label-free analysis of biological specimens using SIMS.
SIMS profiles of (a) PET films, (b) trehalose and (c) peptide obtained from cosputtering
Scanning XPS Microprobe
In an ultra-high vacuum (UHV) chamber (10-7~10-8 Pa), photoelectrons are excited with X-ray (or vacuum UV at 20-40 eV). As the escape depth is shallow, the information came from the top few nm. This is important in studying the outer-most chemical composition and chemical structure of materials. Using microfocused scanning X-ray, 10-1400 µm beam size with 0.5 eV resolution can be obtained. Using He discharge VUV source, 0.125 eV resolution can be achieved. Combine with the integrated sputter gun (Ar+/O2+/Xe+ and/or C60+), one can slowly remove the surface and acquire spectra from different depth to construct the depth profile. The specimen temperature can be controlled between -120 to 500 °C during analysis. In combination with a quadrupole mass analyzer, this system also serves as a dynamic secondary ion mass spectrometer (D-SIMS) for analyzing soft materials.
Training Material for XPS [Lecture for XPS]
Training Material for SIMS [Lecture for SIMS]
Complete course and recording: 01 02 03 04 05 06 07 08 09 10 11
extract spectrum from depth profile
fit for chemical states in depth profile
|PHI TRIFT V nanoToF
Time-of-Flight Secondary Mass Spectrometry
Based on energetic ion bombardment, secondary ions (SIs) are generated. By accelerate SIs with constant 3 kV, ions of different m/z require different times to pass a 2 m flight path and are separated. Using pulsed primary beam of low current, interaction depth of sub-monolayer can be controlled as static SIMS (S-SIMS). With TRIple Focusing Time-of-Flight (TRIFT) spectrometry, >150 µm depth of field, >±20° (up to >±90°) angular solid angle, and ~10000 m/Δm with tunable 20-240 eV pass band can be achieved. Focused Bix+ primary gun (LMIG) provides 70 nm spatial resolution and bunched operation provide best mass resolution. C60+ can also be used for generate larger molecular SI at 2 µm spatial resolution. Integrated Ar+/O2+ and Ar2500+ are also available for sputtering. With specially designed sample manipulation system, specimens can be handled at controlled atmosphere and temperature between -150 to 600 °C can be controlled during introduction and analysis. Selected secondary ion can also be diverted to a tandem linear ToF with or without collision with Ar gas for SIMS/MS analysis.
To minimize the environmental interference to the high resolution work, active field canceling system (Spicer Consulting SC20) is used to control the sum of DC-5 kHz electromagnetic to be <0.07 mG and active vibration isolator (HERZ AVI-200S/M/LP) is used to support the system.
Training Material for SIMS [Lecture for SIMS]
Training Material of ToF [Lecture for ToF]
Complete course and recording: 01 02 03 04 05 06 07 08 09 10 11 12 13
Basic data processing Advanced data processing
Processing of depth profile
Measuring primary beam current and setting for post ESA blanker
Basics of PCA and recording
PCA processing of spectrum with Solo+MIA
3D data transfer 3D data processing Z-correction
find possible structure with chemdraw to assign SIMS peaks
MS/MS operation-1 MS/MS operation-2
|FEI Nova200 NanoSEM
Scanning Electron Microscope (SEM)
Using a electron beam (0.2-30 kV), 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. Dedicate two-channel solid-state detector provides topographic or Z-contrast using back-scattered electrons. Based on stereophonic calculation and images of 2-3 tilt-angles, MeX software provides true 3D information at high resolution. The system is also modified to serve as STEM for BF/DF/HAADF operation. Combine with the X-ray Energy Dispersive Spectroscopy (XEDS, Oxford X-Max 80 mm2 Silicon Drift Detector (SDD), the chemical composition can also be determined in a manner of point-and-click. Quantitative mapping with and without standard is also available. The system also has e-beam lithography capability using Nanometer Pattern Generation System that drives scanning coils according to user designed CAD files.
To provide a stable environment, active vibration isolator (HERZ AVI-200S) is used to support the microscope.
Training Material for SEM (part I)
Training Material for SEM (part II)
Training Material of EPMA
Complete course and recording on SEM: 01 02 03 04 05
Complete course and recording on EPMA: 01 02 03 04
|PHI 690 Scanning Auger Nanoprobe|
Auger Electron Microscope (AES)
Based on a 1-25 kV SEM system with <10 nm beam size, kinetic energy of emitted Auger electrons were analyzed using Cylindrical (Coaxial) Mirror Analyzer (CMA). As the escape depth is shallow, the information came from the top few nm. This is important in studying the outer-most chemical composition and chemical structure of materials. The analyzer normally operated in Fixed Retard Ratio of 0.5% and can achieve 0.05% with sample bias. Combine with the integrated Ar+ sputter gun, one can slowly remove the surface and acquire spectra from different depth to construct the depth profile. The low energy ion can also be used to alleviate the charge-up effect on insulating samples. An in situ cryogenic fracture mechanism is also available to prepare fresh surface for analysis.
To provide a stable environment, active vibration isolator (HERZ AVI-200S) is used to support the microscope.
Complete course and recording on Auger: 01 02 03
|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 (<nm). The usual operation modes include scanning tunneling microscopy (STM); atomic force microscopy (AFM) at contact, tapping, phase image, conductive, and lift mode; force curve (with k-calibration); I-V curve; lateral force microscopy (LFM); chemical force microscopy (CFM); magnetic force microscopy (MFM); electric force microscopy (EFM); Kelvin probe microscopy (KFM, a.k.a. scanning electric potential microscopy, SEPM); Piezo-Response Microscopy (PRM). The sample can be under air or immsered in liquids using the fluid cell.
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.
Complete course and recording: 01 02 03 04 05 06 07 08
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.
An optical/fluoresce microscopy (Olympus BX51) with Differential Interference Contrast is integrated for concurrent observation.
|Anton Paar SurPASS
Electro-Kinetic Analyzer (EKA)
The instrument is used for measuring the zeta-potential of bulk materials and is complementary to DLS. This zeta-potential is important to predict the stability if colloidal suspensions and the tendency to agglomerate.
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. For polymers, the distribution of molecular weight can also be determined in static light scattering (SLS) mode. 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.
|Cell Culture Facilities
The P2 level (NSF49 approved) cell culture room is equipped with direct-heat CO2 incubator, and optical microscopes with CCD to obtain transmissive/reflective bright-field/fluorescent/phase-contrast images.
Transmission Electron Microscope (TEM), managed by the Core Facilities for Nanoscience and Nanotechnology
The high-resolution electron microscope (HREM) and scanning TEM (STEM) has a resolution about angstrom. Atomic arrangements can be observed directly with this instrument. An Oxford X-Max 80 mm2 Silicon Drift Detector (SDD) is installed for XEDS analysis. Gatan Image Filter (GIF) Tridiem is installed for electron energy loss spectrometry (EELS) and energy-filtered TEM (EFTEM)
Course in Electron Microscopy
|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.