NaYF4:Er3+ for flexible SWIR photodetectors

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Au NRs for SWIR photodetectors

In this chapter, we aim to achieve SWIR photodetection by colloidal Au NRs with an optical absorption cross-section covering SWIR spectrum. It is realized by obtaining suitable Au NRs with their absorption tunable in the visible-SWIR spectrum by the tunability of their aspect-ratio. In order to harvest the remarkable photothermal effect from these Au NRs, we combined them with thermistors and resistance temperature detectors (RTD) to form hybrid photodetectors. Introduction of the characteristics and feasibility of using Au NRs for SWIR photodetection is performed in chapter 2.1. Experimental details of synthesis, device fabrication and measurement are performed in chapter 2.2. Then, we present the results and discussion in chapter 2.3.

Introduction of Au NRs

In recent decades, gold nanoparticles, especially gold nanorods (Au NRs), have aroused extensive concern in the application of biomedical field, optical sensing and optoelectronic devices owing to their extremely attractive properties: (i) surface plasmon resonance (SPR) dominant optical properties, (ii) tunability of the rod aspect-ratio (AR) and therefore optical spectra in the visible-SWIR region, (iii) high physical & chemical stability and low biological toxicity.26–30 Due to these fascinating properties, a great deal of research and literature has emerged about Au NRs synthesis,48–50 self-assembling,51–53 coupling modes54–56 and optical applications, such as light scattering,57,58 photothermal therapy,59–62 photoluminescence,63 nanothermometers64 and pressure sensor65.
Owing to the tunability of the aspect-ratio (AR), Au NRs can give optical spectra in the visible-NIR-SWIR region,66 meaning that they have potential detectivity to form broadband photodetectors, especially for SWIR (1000 nm – 3000 nm) photodetection, which mainly relies on InGaAs,67,68 InSb,69 PbS,70 and HgCdTe.71 To our best knowledge, vast amounts of photo-sensing applications of Au NRs appeared, mainly in biomedical field, in vivo imaging72–74 and photothermal cancer therapy.75–78 But most of current research focus on the optical region < 1000 nm (AR < 6): Aquiles Carattino et al used gold nanoparticles as absolute nano-thermometers (AR = 2),64 Edakkattuparambil Sidharth Shibu et al applied small gold nanorods for photothermal microscopy in cells (AR = 2 ~3),79 Hui Hou et al fine-tuned the localized surface plasmon resonance (LSPR) of Au NRs with high photothermal efficiency for cancer cell ablation (AR = 3 ~4),75 Takuro Niidome et al modified Au NRs for in vivo applications (AR = 6)80… Nevertheless, for high aspect-ratio Au NRs (AR > 10, LSPR covering of SWIR region), the discussion and application are still rare. Applying Au NRs for SWIR photodetection has not been presented or implemented, to our best knowledge.
Figure 2.1 Photoexcitation and relaxation of metallic nanoparticles. a–d, Photoexcitation and subsequent relaxation processes following the illumination of a metal nanoparticle with a laser pulse, and characteristic timescales. a, First, the excitation of a localized surface plasmon redirects the flow of light (Poynting vector) towards and into the nanoparticle. b–d, Schematic representations of the population of the electronic states (grey) following plasmon excitation: hot electrons are represented by the red areas above the Fermi energy EF and hot hole distributions are represented by the blue area below EF. b, In the first 1–100 fs following Landau damping, the thermal distribution of electron–hole pairs decays either through re-emission of photons or through carrier multiplication caused by electron–electron interactions. During this very short time interval τnth, the hot carrier distribution is highly non-thermal. c, the hot carriers will redistribute their energy by electron–electron scattering processes on a timescale τel ranging from 100 fs to 1 ps. d, Finally, heat is transferred to the surroundings of the metallic structure on a longer timescale τph ranging from 100 ps to 10 ns, via thermal conduction. [Ref. 41]
Meanwhile, the hot carriers generation and transfer in plasmonic materials create a feasible condition for fast response photodetection.30,41,81,82 Generation and transfer processes of hot carriers are shown in Figure 2.1.41 After photon absorption and subsequent LSPR excitation in Au NRs, hot carriers are generated via Landau damping on a time scale ranging from 1 to 100 fs, then quickly redistribute their energy toward lower energy electrons via electron-electron scattering followed by thermalization with the lattice and heat dissipated to the surroundings on a time scale ranging from 100 ps to 10 ns. Subsequent heat transfer is related to the restriction from the nanoparticle size, the material and the thermal conduction properties of the surroundings.30,41
This ultrafast process of hot carriers generation and transfer in plasmonic materials reveals that metallic nanoparticles (e.g. Au NRs) have effective photosensitive property. Considering their large optical absorption cross-section and tunability of their optical spectra26–30, colloidal Au NRs can be an alternative material for photodetectors. Therefore, in this project, a series of monodisperse colloidal nanocrystals were synthesized in Experimental section 2.2.1 by modifying the silver-assisted seed-mediated growth method applying binary surfactants reported previously.48 NaYF4 :Er3+ fluorescent nanocrystal were synthesized (Experimental section 2.2.2) to prove that the photothermal effect from Au NRs is high-efficiency, then two hybrid device structures, composing of Au NRs on the surface of a NTC-thermistor and Platinum wire, were fabricated and measured in Experimental section 2.2.3 and Experimental section 2.2.4, respectively. These hybrid devices show an alternative low-cost approach for sensing in the SWIR spectrum (results and discussion in chapter 2.3).

Experimental section

Synthesis of colloidal Au NRs

Au nanoparticle seed solution: After mixing 5 ml of 0.5 mM HAuCl4 solution with 5 ml of 0.2 M CTAB solution, 1 ml of 0.006 M NaBH4 solution was added followed by vigorously stirring for 2 minutes. This solution was then left un-stirred for aging during 30 minutes.
Au NR growth solution: 9 g of CTAB was evenly mixed with different amounts of NaOL (listed in Table 2.1). They are then dissolved in 250 ml of de-ionized water under stirring at 50 oC. After the solution was cooled down to 30 oC, different amounts of 4.15 mM AgNO3 solution were added and the whole solution was kept at 30 oC for 15 minutes. This is followed by the addition of 250 ml of 1 mM HAuCl4 solution under stirring for about 90 minutes (until the solution turned colourless). At this stage, different amounts of a 12.1 M HCl solution were added under stirring for 15 minutes. 1.25 ml of 0.064 M ascorbic acid solution was then added under vigorous stirring for 30 s. Finally, different amounts of seed solution were added first under rigorous stirring for 30 s then the whole solution was kept un-stirred for 12 hours at 30 oC for NR growth. After 12 hours of growth the final product was isolated by centrifugation (6k rpm for 30 minutes) and subsequent decantation of mother liquid. De-ionized water was then added to re-dissolve the Au NRs. The centrifugation/decantation process was repeated for 3 times and the final Au NR products were dissolved in de-ionized water.
2.2.2 Synthesis of NaYF4:Er3+ fluorescent nanocrystals
To demonstrate the photothermal effect of these Au NRs, a nanothermometer, NaYF4 nanocrystal (NC) doped with Er3+ ions were synthesized by modifying the typical hydrothermal method described in ref 83. The relative composition was 85% of Y and 15% of Er. The nanocrystal had a hexagonal phase and their diameter was around 230 nm.
In a typical preparation of NaYF4: 15 mol % Er3+ NC, a 5 mL aqueous solution of Ln(NO3)3 •6H2O (0.2 mol/L, lanthanide ion molar ratio, Y: Er = 85:15) was mixed with a 5 mL aqueous solution of sodium citrate tribasic dihydrate (1.8 mol/L) under vigorous stirring. A white precipitate of lanthanide citrate was observed. A 5 mL aqueous solution of NaF (2.4 mol/L) was then added slowly into the mixture. After being stirred for 1 h, the resulting precursor solution was transferred to a 70-mL-volumn autoclave. The autoclave was then placed in a digital type temperature controlled oven and heated at 120 oC for 2 hours. The autoclave was then allowed to cool down to room temperature naturally (typically overnight). The precipitate in the autoclave (NaYF4: Er3+ NCs) was then separated from the reaction media by centrifugation (6000 rpm, 30 min), decantation, and dispersion in DI water. This purification process was repeated three times. The final synthetic product was first dried in a vacuum oven at 60 °C for 24 h before being annealed in air at 300 °C for 2 hours.

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Fabrication and measurement of Au-NRs/Thermistor

To harvest the photothermal from the Au NRs, a temperature- sensitive thermal device, a negative temperature coefficient (NTC) thermistor, was introduced to fabricate a hybrid Au-NRs/Thermistor device. This NTC thermistor with a room-temperature (25C) resistance of 10 k (Series B57540G1 from TDK EPCOS) was obtained from RS components (no. 769-1892, 3 €/each). It is then fixed on a glass microscopy slide. To fabricate the hybrid Au-NRs/thermistor device, a drop ( 3 µL) of Au NR water solution (sample h of table 2.1, with a concentration ranging from 0.6 g/L to 7.2 g/L) was casted directly to coat the surface of the thermistor. After the Au NR solution was dried on the thermistor, Scanning Electron Microscopy (SEM) characterizations were performed directly onto the hybrid device (FEI Magellan 400 system, field emission gun source). The hybrid device was measured under monochromatic illumination generated by a quartz halogen lamp and a computer-controlled Oriel Cornerstone monochromator (and appropriate order sorting filters). The intensity of the monochromatic illumination was calibrated by a NIST-calibrated germanium photodiode. The monochromatic illumination was modulated by a shutter while we measured the resistance of the hybrid and control device by a computer-controlled Keithley 2634B source measurement unit (SMU). Concerning measurements under laser, the output of a fiber-coupled laser diode ( = 1.5 µm) was focused onto the hybrid or control device. Neutral density filters were used to adjust the laser power while the laser power was calibrated by a laser power meter. The laser illumination was modulated by a shutter while we measured the device resistance by a Keithley 2634B SMU.

Fabrication and measurement of Au-NRs/Pt device

To harvest the photothermal from the Au NRs, a platinum (Pt) microwire was introduced to fabricate a hybrid Au-NRs/Pt device. The Pt microwire resistance temperature detector (RTD) devices with a series of width were prepared by laser direct writing lithography (Heidelberg Ins. Laser writer µPG 101). Quartz coated glass substrates were cleaned by ultrasonication in a bath of de-ionized (DI) water, acetone and 2-propanol followed by O2 plasma etching. A layer of adherence promoter (TI from microchemicals) and a layer of photosensitive resin (AZ5214, microchemicals) were spin-coated one after the other on these glass substrates. Pt microwires of various dimensions were then defined by direct laser writing lithography (Heidelberg Ins. laserwriter µPG101). Substrates were then developed by dipping the sample in a bath of developer (AZ726 MIF, microchemicals) for 30 s with subsequent DI water rising and air-drying. A Ti (5 nm)/Pt (40 nm) layer was then evaporated onto the sample by an e-beam evaporator (Plassys MEB550S). Finally, the sample was lifted-off by immersing samples into a bath of acetone.
To fabricate the Au-NRs/Pt hybrid device, a drop ( 1 µL) of Au NRs water solution (with a concentration of 1 g/L – 6 g/L) was casted directly to coat the surface of Pt microwire. After the Au NR solution was dried on the surface, SEM characterizations were performed directly onto the hybrid device (FEI Magellan 400 system, field emission gun source). The hybrid device was measured under monochromatic illumination generated by a quartz halogen lamp and a computer-controlled Oriel Cornerstone monochromator (and appropriate order sorting filters). The intensity of the monochromatic illumination was calibrated by a NIST-calibrated germanium photodiode. The monochromatic illumination was modulated by a shutter while we measured the resistance of the hybrid and control device by a computer-controlled Keithley 2634B source measurement unit (SMU). Concerning measurements under laser, the output of a fiber-coupled laser diode ( = 1.5 µm) was focused onto the hybrid or control device. Neutral density filters were used to adjust the laser power while the laser power was calibrated by a laser power meter. The laser illumination was modulated by a shutter while we measured the device resistance by a Keithley 2634B SMU.

Characterization for materials

Agilent Cary 5E UV-Visible-NIR spectrometer was used for Au NRs’ UV-Visible absorption spectra measurements, as shown in the following figures: Figure 2.2 Absorbance spectrums of Au NRs in water.

Results and discussion

Aspect-ratio tunability of colloidal Au NRs

Here, by a careful synthetic tuning, we synthesized a series of monodisperse colloidal gold nanorods of different aspect-ratios (ARs) with their longitudinal LSPR maximum tunable from 900 nm to 1.3 µm. These gold nanorods (Au NRs) with different ARs were synthesized by modifying the silver-assisted seed-mediated growth method applying binary surfactants reported previously.48(Synthesis detail is shown in Experimental section 2.2.1) While most reported colloidal Au NRs exhibit a longitudinal LSPR (L-LSPR) in the wavelength range shorter than 1200 nm,84–88 we successfully extend the aspect-ratio (AR) of Au NRs up to 10.96 with their L-LSPR absorption peak reaching 1300 nm. Such a synthesis involved first the formation of gold nanoparticle seeds by reducing gold (III) chloride (HAuCl4) by sodium borohydride (NaBH4) in the presence of hexadecyltrimethylammonium bromide (CTAB) as surfactant. Different amounts of Au seeds were further placed into a growth solution where Au(III) is slowly reduced to Au(I) by a combination of ascorbic acid (AA) and sodium oleate (NaOL) in the presence of hydrochloric acid (HCl), CTAB and silver nitrate (AgNO3). In such a multi-parameter synthesis, we have specifically experimented various amounts of Au seeds, AgNO3, surfactant NaOL, and the acidity of the growth solution (amounts of HCl) (summarized in Table 2.1).
Comparing the results of sample c and sample e (Table 2.1), we observed an increased average length and diameter of Au NRs associated with the decreased amount of Au seeds, which is possibly due to more growth material available per seed. Sample e exhibited a larger AR compared to sample c due to their smaller NR width (diameter). The role of Ag3+ in such a synthesis has been reported as crucial as it interacts with CTAB to form elongated templates and hinders the growth of certain crystallographic facets of Au NRs.26,89 Increasing the amount of Ag3+ from the optimum value however led to a decrease of AR due to the thickened NR width (comparing sample b and c, Table 2.1). Concerning the amount of surfactant NaOL, we observed longer and thicker Au NRs with larger AR when the amount of NaOL decreases from 1.234 g to 0.925 g (sample f and g, Table 2.1). The use of NaOL here together with CTAB in the growth solution represents a binary surfactant strategy. NaOL serves here both as a reduction agent and likely also as a surface binding mediator between CTAB and certain facet of Au to allow NR morphological tuning. Concerning HCl, one modifies the NR growth by controlling the acidity of the growth solution. A larger amount of HCl here led to a reduction of pH favoring larger AR by reducing more significantly the width of NRs (sample g and h, Table 2.1). By controlling these four parameters we were able to tune the AR of NRs from 4.47 to 10.96. As the L-LSPR of Au NRs are highly sensitive to their AR, this series of samples thus exhibits distinctive optical property with a strong and well-defined L-LSPR progressively red-shifted towards 1300 nm as AR increases (Figure 2.2).

Table of contents :

1. Introduction
2. Au NRs for SWIR photodetectors
2.1 Introduction of Au NRs
2.2 Experimental section
2.2.1 Synthesis of colloidal Au NRs
2.2.2 Synthesis of NaYF4:Er3+ fluorescent nanocrystals
2.2.3 Fabrication and measurement of Au-NRs/Thermistor
2.2.4 Fabrication and measurement of Au-NRs/Pt device
2.2.5 Characterization for materials
2.3 Results and discussion
2.3.1 Aspect-ratio tunability of colloidal Au NRs
2.3.2 Photothermal effect of Au NRs
2.3.3 Au NRs/NTC-thermistor photodetector
2.3.4 Au-NRs/Pt photodetector
2.4 Summary
3. NaYF4:Er3+ for flexible SWIR photodetectors
3.1 Introduction of upconversion nanoparticle
3.2 Experimental section
3.2.1 Synthesis of NaYF4:Er3+ UCNPs with tunable sizes
3.2.2 Synthesis of ZnO nanoparticles
3.2.3 Device preparation and characterization
3.2.4 Structural and optical characterizations
3.3 Results and discussion
3.3.1 Size-tunability of NaYF4:Er3+
3.3.2 Optical property of UCNPs and UCNP-organic hybrids
3.3.3 Device architecture and photoresponse performance
3.3.4 Comparison with other SWIR photodetectors
3.3.5 Mechanical bending tests on flexible hybrid devices
3.4 Summary
4. Conclusion and perspectives
5. Annex
5.1 Annex I. supporting information for Au-NRs/thermistor
5.2 Annex II. Supporting information for Au-NRs/Pt device
5.3 Annex III. Supporting information for UCNPs/Polymer device
6. Publications
7. References

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