In this chapter, we will introduce the experimental systems and techniques including Micro-Raman spectrometer, fluorescence lifetime image, the Scan Near filed Optical Microscope (SNOM), and the reflectance spectrometer system.
§ 2-1 Micro-Raman Spectrometer
The Micro-Raman spectrometer (Jobin Yvon LabRam HR800 [1]) is used during our photo luminescence (PL) experiments (illustrated in chapter 4) and Surface Enhanced Raman Scattering (SERS) experiments (illustrated in chapter 7). Fig. 2.1 describes the simplified optical path of the apparatus [1]. He-Ne (633nm) laser is set as the incident source for the PL (and the SERS) measurement, the objective we used is (and for the SERS ), and the laser power through the density filter is down to ~3 mW (and for the SERS ~ 0.45 mW). After the excitation laser spot is focused (in the normal direction) on the sample (for fluorescence PL: spin-coated the highly dilute quantum dots (QDs) solution on the Au caps on silicon nanowires (SiNWs), and for the SERS: dilute crystal violet (CV) aqueous solution on the Au coated silicon nanorads (SiNRs)). The entrance filter is for purifying the laser source, while the notch filter is for filtering out the laser and getting the fluorescence PL (and the SERS) signals from the samples. The fluorescence PL (and the SERS) signals pass through the notch filter and go to the turning grating, and then collected by the charge couple device (CCD) and translated to electric signals for analyzing. In Figs. 2.2-2.3 [
×100
×50
1], pictures of the apparatus side view I and II are shown.
Fig. 2.1 Optical path simple description of the Micro-Raman spectrometer. M1, 2, 3, 4, 5 are mirror1, 2, 3, 4, 5, and obj is the objective. The minimum step of the piezo sample stage is 20 nm [1].
Fig. 2.2 Jobin Yvon LabRam HR800, side view I [1]
Fig. 2.3 Jobin Yvon LabRam HR800, side view II [1]
§ 2-2 Fluorescence Lifetime Imaging Microscopy
The fluorescence lifetimes of the QDs on the different sample surfaces used in our experiments (illustrated in chapter 4 and chapter 6) were measured by the apparatus shown in Figs. 2.4-2.5 [2].
The excitation laser (picosecond pulsed diode laser, repetition frequency: 10 MHz, wavelength 405 nm, power ~ 0.13 μW , pulse width 50 ps) directed by an optical fiber is led into the main optical unit and collected by a collimating lens, and then reflected by a mirror. After the laser reaching the partly reflected/ partly transmitted (R/T) mirror, it is partly reflected to a photo diode which is used for measuring the excitation laser power, and partly transmitted and then reflected by the dichroic mirror and led into the objective (Olympus UPlanSAPO 100xoil, NA = 1.4) connected to a computer controlled piezo-scanner (spatial resolution: nm precision) in the microscope (Olympus IX71). After
the excitation laser spot focused on the QDs, it possibly generates an electron-hole pair.
After the excitation (in an order of ~ ns), the electron-hole pair may have a chance to recombine and emit fluorescence.
The fluorescence collected by the same objective is led into the main optical unit. In the main optical unit, the dichroic mirror which is used for reflecting the excitation laser and letting the fluorescence passing by. The excitation laser is reflected to reach the R/T mirror again, and then led into the charge-coupled device (CCD), which is used for monitoring the laser focus pattern. The fluorescence goes through a long pass 500 nm filter, and then passes through a pin hole (50 μm) and is expanded by two lenses. Finally, the expanded fluorescence goes through a filter for purifying and then focused by a lens before reaching a single photon avalanche photodiode (SPAD, response time is about 400 ps) which turns the light signal into the electric signal for TTTR (Time-Tagged Time-Resolved) analysis.
Time-Correlated Single Photon Counting (TCSPC) histogram of QDs fluorescence is obtained from TTTR provided by PicoQuant. Every 100 ns, there is a laser pulse impinges on the target. After 60 s, there are excitation cycles which produce
counts of fluorescence (for our QDs samples). These are abundant for statistic. TCSPC histograms are formed by recording the correlated time (the excitation laser trigger time correlated to single photon arrival SPAD time) of each single photon produced in each excitation cycle, and accumulating single photon numbers in a bin time for all cycles.
According to TCSPC histograms, some of the fluorescence intensities of the QDs can be nicely fitted by single exponential decay: . Where is the intensity (counts) at the time , and representing the number of photons (given by counts)
Fig. 2.4 The schematic diagram of fluorescence images and the lifetime measurement system [2]. R/T Mirror is the reflection/transmission mirror.
Fig. 2.5 The fluorescence images and the lifetime measurement system [2]
§ 2-3 Scanning Near-Field Optical Microscope
The idea for SNOM (also named Near-Field Scanning Optical Microscope (NSOM)) was first proposed by E. H. Synge in 1928 [3]. An imaging instrumnet originated from the concept of exciting and collection diffraction in the near field was first developed by Ash and Nichols in 1972 [4], the diffraction limit (expressed by the Rayleigh criterion:
d NAλ
NA= is the numerical aperture for the optical component. is the index of refracion of the medium where the lens is working in, and
n
θ is the half-angle of the maximum cone of light incident into the lens) was first broken [5]. Later, Pohl et al. and Lewis et al. developed a NSOM (resolution can reach ~ λ/20) with a metal coated aperture at the tip of a sharp probe, and a feedback system[6-7].
Instrument picture of the NSOM (Aurora-3), schematic of the light paths, tuning fork mechanism and the AFM tip are shown in Fig. 2.6 [8]. The Excitation laser source in an NSOM system is led into an optical fiber. The end of the fiber sharpened to a diameter of ~ 50 nm and coated with aluminum (approximately 100 nm thick) as a tip. A scanning probe microscope (SPM) equipped in an NSOM is comprised of a sensing probe (for scanning across the sample surface), piezoelectric ceramics (for positioning the sample), an electronics control unit and a computer (for controlling the scan parameters and generating images). A SPM has provided the technology needed to maintain the tip-sample spacing (typically less than 10 nm) while a tip scanning over a sample. Two modes (with different relative positions of photo detectors), transmission mode and reflection mode are available for different sample types in the SPM. Photo detectors are placed behind the sample (or beside the tip) for transmission mode (or for reflection mode) to collect light emitted from the sample. In the experiment illustrated in chapter 5, we use the reflection mode. A sensor
(with very high spatial resolution, and can sense height changes ~ 0.1 Ao ) is another important component in the SPM. There are two imaging modes (defined by the types of the sensor): Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) in the SPM. In our experiment (illustrated in chapter 5), the Aurora-3 operates in AFM non-contact mode and uses a tuning fork mechanism (a tuning fork mounted alongside the tip and made to oscillate at its resonance frequency) as a sensing probe. The AFM and NSOM images are simultaneously obtained. The excitation light is locally illuminated by a nano-aperture (~80 nm), the diameter of the fiber tip is about 250 nm, and the near-field signals were collected by an objective at the 45° normal to the sample surface.
Fig. 2.6 Instrument picture of the scan near field optical microscope (SNOM), schematic of the light paths, tuning fork mechanism and the AFM tip (very close to the sample < 10 nm) are shown. The AFM image and the laser light reflection were obtained simultaneously.
[8]
§ 2-4 Reflectance Spectrometer
Fig. 2.7 The schematic of optical system of the reflectance spectrometer [9]
The spectrometer (Model U-3010 [9]) is used for reflectance analysis of liquid, solid and gaseous samples in the ultraviolet-visible region (wavelength: 190 nm ~ 900 nm). Our relative experiments (Au coated SiNR array structures reflectance spectra) will be illustrated in chapter 5, 6 and 7. The optical system of the spectrometer is shown in Fig. 2.7 [9]. A light from the light source (the tungsten iodide (WI) lamp: in the visible region; the Deuterium (D2) lamp: in the ultraviolet region) is selected automatically by the light source switching mirror (optionally selectable in a range of 325 to 370 nm) according to the measurement wavelength. Then, the selected light passed a filter, reflected by a spherical mirror, passed an entrance slit, and let to the monochromator. The monochromator (with variable pitch stigmatic diffraction grating; the grating constant: 1/600 mm) is employing
unique stigmatic concave grating. After passed an exit slit, the monochromatic beam is branched into the reference beam and the sample beam (the reference beam and the sample beam are included in the sample compartment) by a group of sector mirrors (including two plane mirrors, two toroidal mirrors, and a rotating mirror). The beams which have passed through the sample compartment are let by two toroidal mirrors and two plane mirrors, and finally irradiated into the detector (the Photomultiplier tube (PMT)). The reflection value is obtained from the intensity value from the sample side/ the intensity value form the reference side.
Reference:
[1] HORIBA JOBIN YVON, HR800 User manual [2] http://www.picoquant.com
[3] http://www.nanonics.co.il/index.php?page_id=149A [4] Ash E.A. and Nicholls G. Nature 237, 510 (1972)
[5] http://en.wikipedia.org/wiki/Near-field_scanning_optical_microscope
[6] Lewis A., Isaacson M., Harootunian A., and Murray A. Ultramicroscopy 13, 227 (1984)
[7] Pohl D.W., Denk W., and Lanz M. Appl. Phys. Lett. 44, 651 (1984) [8] AURORA-3 Instrument User Guide
[9] Instruction manual model U-2010/U-3010/U3310 spectrometer (maintenance manual), Hitachi-High Technologies Corporation 2001, 6th Edition (2005)