Apostolos
Segkos
*ab,
Ilias
Sakellis
a,
Nikolaos
Boukos
a,
Charalampos
Drivas
c,
Stella
Kennou
c,
Konstantinos
Kordatos
b and
Christos
Tsamis
a
aInstitute of Nanoscience and Nanotechnology (INN), National Centre for Scientific Research “Demokritos”, Patr. Gregoriou E & 27 Neapoleos Str., Aghia Paraskevi, 15341, Athens, Greece. E-mail: a.segkos@inn.demokritos.gr
bDepartment of Chemical Sciences, School of Chemical Engineering, NTUA, 15780 Athens, Greece
cDepartment of Chemical Engineering, University of Patras, 26504, Patras, Greece
First published on 1st May 2020
Carbon dot-based fluorescent nanocomposite compounds were obtained following microwave assisted thermal treatment of an aqueous mixture consisting of citric acid and urea. Thin film deposition of nanocomposites on SiO2 (100) substrates is followed by annealing, in order to render the films dissolution-resistant and processable. Optical lithography and O2 plasma etching are utilized to pattern the deposited films in the desired shapes and dimensions and a solid-state relative humidity sensor is fabricated on the SiO2 substrate. Spectroscopy and microscopy techniques are employed to characterize and monitor the whole process throughout the fabrication steps. The patterned films retain the functional groups introduced during their synthesis and continue to display hydrophilicity and PL properties. Successful patterning of these nanocomposites opens the way for the fabrication of solid-state, carbon dot-based optical and electrical devices that take advantage of the properties of carbon quantum dots.
Research on these carbon nanomaterials has especially intensified following the report from Xu et al. on a new group of fluorescent nanoparticles, named carbon quantum dots (CQDs), in 2004.1 These novel nanoparticles possess optical properties comparable to their semiconductor counterparts, namely high quantum yield and resistance to photobleaching;2,3 while also exhibiting low toxicity, they can be effortlessly synthesized following green low-cost synthesis routes5,6 and have proven to be an excellent template for further functionalization and engineering of their surface chemistry. To date, various approaches for the synthesis of CQDs have been reported, generally grouped and referred to as “bottom-up” and “top-down” routes.7 Of particular interest are the hydrothermal and microwave assisted bottom-up approaches since they have proven to be facile, quick and low-cost synthesis routes which allow the simultaneous one-pot synthesis and functionalization of the CQD surface chemistry by choosing appropriate precursors.8–10 For the bottom-up synthesis of CQDs, citric acid (CA) and urea (UR) have become popular precursors, due to their relatively low cost. Citric acid acts as the carbon backbone donor, whereas urea provides nitrogen containing groups, which have been proven to improve the quantum yield (QY) of CQDs.11–13
To date, CQDs have been employed in various applications including chemical sensing,14,15in vivo cell imaging,16,17 light emitting diodes,18,19 photocatalysis,20,21 solar cells22,23 and more, and exploited in the form of suspensions, inks, thin films, and as scaffolds or as parts of heterostructures. For solid state applications, in particular, CQDs have been used in conjunction with polymeric matrices, either being dispersed in or sandwiched between them,18,24–26 thus creating fluorescent nanocomposites. In addition, some authors have created fluorescent patterns using CQDs as pen inks, or by ink-jet printing and silk-screen printing techniques.2,27–29
Although these techniques allow transferring of CQDs from a suspension to solid state films and the creation of patterns, they lack in resolution and process control as opposed to widespread microfabrication techniques such as photolithography, especially when it comes to device miniaturization. Control over deposition and patterning parameters is critical in order for a functional material to become a processable building block for MEMS/NEMS fabrication. Weng et al. have reported the synthesis of individual CQDs with controlled position in nanoscale terms by the in situ formation of CQDs on poly(2-(methacrylamido)glucopyranose) (PMAG) by e-beam irradiation of PMAG nanostructures.30 While the reported control of the created patterns and position of CQDs is very precise, it requires expensive equipment and is time consuming for larger scale patterning and production. In this work, we report a methodology for the deposition of fluorescent CQD-based nanocomposite thin films on SiO2 substrates and their subsequent manipulation, in order to obtain dissolution resistant (immobilized), lithographically patterned films that retain their fluorescent properties. Taking a further step forward, we utilize this process to fabricate miniaturized devices on SiO2 substrates and characterize their electrical properties as well as their response as humidity sensors. The fluorescent nanocomposites were synthesized following two different bottom-up routes. They exhibited high hydrophilicity and were handled in the form of aqueous suspensions. The materials were deposited as thin films on SiO2 substrates and were further thermally annealed to obtain dissolution resistant films. By ensuring that the films were stabilized upon substrate post-treatment – i.e. not being washed away by solvents commonly used in microelectronic fabrication techniques, such as water, acetone or iso-propanol, even under ultrasound treatment – we were able to create multi-layered assemblies and followed by photolithography to pattern the deposited films. Finally, relative humidity sensors were fabricated by combining the abovementioned processes on films patterned between interdigitated electrodes (IDEs).
Transmission electron microscopy (TEM) studies were performed on CQDa. These studies confirmed that the as-synthesized CQDa is indeed a nanocomposite material, comprising a polymer-like material, in which graphene strips and quasi-spherical carbon quantum dots with a diameter in the range of 2–6 nm are dispersed.
More specifically, in Fig. 1A evidence of the amorphous polymeric by-product can be seen, especially at the edges of the droplet on the TEM grid. In Fig. 1C, the graphene strips are shown, with their (0002) crystallographic plane in view. The graphene strips have lengths of 10–20 nm and approx. 5 nm thickness. Finally, in Fig. 1D individual carbon dots are shown, while a typical distribution of the dots within the polymeric matrix can be seen in Fig. 1B. Furthermore, fast Fourier transform revealed d-spacing ascribed to graphitic (0002) and (1010) crystallographic planes, thus verifying the graphitic core of CQDs. The obtained compounds were easily dispersed in polar protic solvents such as water and ethylene glycol and were spin-coated on SiO2 wafers that had been previously cleaned with piranha solution to become more hydrophilic.
Fig. 1 TEM images of CQDa at different magnifications. Scale bars are (A) 100 nm, (B) 20 nm, (C) 10 nm and (D) 5 nm. |
In order for the films to be processable and to enable patterning, they were required to be dissolution resistant so that they could withstand processes involving washing with common solvents such as water, acetone and iso-propanol. To this end, the films were first heated on a hotplate at 120 °C for a mild removal of the solvent, before they were subjected to thermal annealing in the range of 200–600 °C in a N2 atmosphere. For all types of CQDs, annealing of the film samples at 200–300 °C resulted in the films being easily washed away, partially or fully, with only a few drops of water, while those annealed at 400–500 °C appeared unaffected after washing with water, acetone or iso-propanol (even when an ultrasonic bath was used), indicating that immobilization of the films was successful. These were the conditions that were used in the next steps for patterning using optical lithography. No film was retained on the samples that were annealed at 600 °C, possibly due to decomposition of the films.
The thickness of the patterned films, for both CQDa and CQDp, was verified by atomic force microscopy following non-contact (NC) and intermittent contact (IC) topography measurements. The single layered samples were determined to be between 1.8 nm and 5 nm, while the three-layered samples were measured to be in the range between 12 nm and 20 nm (Fig. 2). The RMS roughness of the films and that of the substrate were calculated as follows: scanning for various sizes (typically 50 μm × 50 μm, 60 μm × 60 μm, 70 μm × 70 μm and 80 μm × 80) was performed for each sample. From these, representative areas of approx. 10 μm width and of length as depicted in the x-axis were selected (Fig. 3), in order to also compensate for measurement artifacts attributed to the measuring equipment as well as to ambient measurement conditions (e.g. ambient relative humidity and temperature). The roughness per line scanned by the AFM tip was measured and a sample-wide average RMS roughness was calculated. RMS roughness of the single-layer films of both types of deposited fluorescent films was estimated to be approximately 0.3–0.6 ± 0.075 nm (Fig. 3A), while for the triple layer it was found to be 6.18 ± 2 nm (Fig. 3B). A comparison with the roughness of the substrate (0.092 ± 0.019 nm) reveals that the initial layers closely follow the roughness of SiO2, confirming the compatibility of the materials and the suitability of SiO2 as a substrate for device fabrication using the synthesized materials.
Fig. 2 AFM topographies and step height measurements with the respective fittings for single layer CQDa films (A and B), single layer CQDp films (C and D) and three layer CQDa films (E and F). |
In order to extract further information regarding the chemical composition of the CQDs and to observe the changes that thermal annealing confers to the chemical bonds of the respective thin films, FTIR spectra of film samples were recorded before and after annealing (Fig. 4). A comparison of the spectra of the as deposited films of the two types of CQDs reveals small differences. While the spectra cannot be compared quantitatively, it is evident that the same types of functional groups can be found in both materials, albeit with different contributions. For both types of CQDs before annealing, broad absorption bands at 2500–3800 cm−1 were obtained. These are assigned to v(O–H) and amine v(N–H). In addition, the existence of carboxylic CO (1720 cm−1 and 1370 cm−1), carbonyl (1700 cm−1), amine (1400 cm−1) and amide (1330 cm−1) moieties can be verified. After annealing at 400 °C for 10 minutes, the obtained spectra display broad absorption bands at 3100–3500 cm−1, assigned to hydroxyl, carboxyl ν(O–H) and amine ν(N–H) as well as aromatic ring v(C–H), which were previously overlapped by the strong solvent v(O–H) peaks. The contribution of C–N and N–H decreases (1200–1400 cm−1), while CO, C–O–C, CC and –OH bonds appear to be practically unaffected. The above data reveal that, even after heating at 120 °C for 10 minutes, the samples before further annealing appear to contain a considerable amount of absorbed solvent. In addition, it appears that both synthesis approaches result in two types of CQDs with closely related chemical structures. Last but not least, thermal annealing in the range of 400–500 °C in a N2 atmosphere was found to lead to almost identical spectra for both types of CQD thin films: temperature affects the type of nitrogen bond, while having little impact on most other organic functional groups and bonds.34–37
The effect of annealing temperature on nitrogen-containing moieties was also studied by X-ray photoelectron spectroscopy (XPS) performed for three different CQD film samples (Fig. 5): as-deposited; annealed at 400 °C; and annealed at 500 °C. All the wide scan XP spectra exhibited the expected peaks of C, N and O. Deconvolution of the N 1s peak as proposed by Marton et al.38 reveals that nitrogen enters the carbon structure mainly as pyridinic (398.7 eV) and pyrrolic (400.0 eV) groups, and, to a lesser extent, as N–N/N–O (402.0 eV).39,40 Of particular interest is the fact that an increase of the annealing temperature leads to an increase of the pyridinic-to-pyrrolic nitrogen ratio. The data confirm that oxygen and nitrogen containing functional groups are still present even after annealing, although their bonding to the carbon lattice may change or certain forms of these groups may escape as the annealing temperature increases.
Fig. 5 XPS spectra of CQDa thin films – N 1S peak deconvolution for (A) as-deposited, and (B) after annealing at 400 °C and (C) 500 °C. |
In order to verify the optical properties of both the as-synthesized nanocomposite material and the post-annealed thin films, UV-Vis absorption and PL emission spectra were recorded from aqueous dispersions and thin films, respectively. To date, the PL origins of CQDs remain unclear, and are most commonly attributed to a synergistic effect of multiple optically active centres. These centres are thought to be the graphitic core of CQDs, surface and molecular states, as well as edge defects. The core states arise from conjugated π nanodomains or the quantum confinement effect of their graphitic core, the surface states refer to the functional groups attached to the carbon lattice and backbone and the molecular states refer to fluorescent molecules that either coexist or are embedded within or connected to the surface of the CQDs (e.g. 1,2,3,5-tetrahydro-5-oxo-imidazo[1,2-a]pyridine-7-carboxylic acid).11,41,42
The absorption spectra of both types of CQDs (Fig. 6A) showed the distinctive 230 nm absorption band which is attributed to a core state that is thought to be well-connected electronically with the surface/edge states.43,44 In addition, both types also displayed the 340 nm absorption band which can be attributed to a combination of interlayer charge transfer transitions of π–π* nature and n–π* transitions of chromophore groups, such as CO, possibly located on the CQD surface.44 In the case of CQDa, absorption bands at 270 nm and at 400 nm were also observed, which can be ascribed to electronic transitions from π (or n) to π* due to CN and OH/COOH electron donating/withdrawing moieties connected to the conjugated system (surface states). In contrast, in CQDp there was no absorption band at 270 nm or 400 nm, however a low intensity, broad absorption appeared at about 675 nm, which could be ascribed to n–π* transitions of surface states.44,45
The emission spectra recorded from dilute aqueous dispersions of CQDa at 325 nm excitation displayed a weak emission shoulder at 435 nm and a stronger emission peak at about 500 nm. When spin-coated on SiO2 and heated at 120 °C on a hotplate to remove excess solvent, the peak at 435 nm was extinct and a red shift of the strong emission peak at 513 nm was observed (Fig. 6B). The emission spectra of both types of as-deposited CQD films are quite similar, with the main emission peak of CQDa being slightly red shifted by 25 nm in comparison to CQDp films (Fig. 6C). After annealing at 400 °C and 500 °C the photoluminescent response of both CQDa and CQDp films, at the same excitation wavelength, changes. Both types of films have almost identical emission spectra, with a double peak appearing at 375 nm–400 nm and another peak at about 360 nm after annealing at 500 °C (Fig. 6D and E). On comparing the emission spectra, it appears that even changes in the immediate environment of the materials, such as removal of the solvent, confer major changes in their optical properties, thus providing strong evidence of the molecular nature of their fluorescent properties.46 It is possible that by removing the solvent, head-to-tail or parallel molecular aggregates form, leading to charge transfer dominated emission.47 In addition, annealing of the films way above the temperature at which they are synthesized could result in changes in their chemical or structural composition, as observed by FTIR and XPS studies, leading to new optical centres, which could explain the appearance of three new emission peaks. While the study of the optical properties of the materials remains inconclusive, the experimental results presented are important because, among other reasons, they verify that the patterned thin films produced in this work remain fluorescent under these specific treatment conditions. In any case, as far as the optical properties are concerned, further study of the underlying mechanisms is imperative, in order to better understand the physics that govern the optical properties during the transition from solution to the solid state, as well as the effect of solvents and annealing temperatures on these mechanisms.
Following the characterization of the patterned CQD-based thin films, the aforementioned techniques were utilized to fabricate solid-state relative humidity (RH%) sensors. Fabrication of these devices serves as a proof of concept for the ability to employ this process to fabricate miniaturized devices based on CQD thin films of well-defined geometries. More specifically, the sensor was developed by patterning a single layer CQDp thin film between Au interdigitated electrodes on a SiO2 substrate.
The response of the developed sensors was evaluated in various ambient relative humidity contents ranging up to 75%, under a 2V bias (Fig. 7A). We notice that for each humidity value a steady state current value is reached
An important observation is the full recovery of the output of the device when returning to 0% RH, showing a response devoid of hysteresis phenomena even in the cases of high humidity levels. The response of the sensor for the various RH% contents was fitted using an exponential function (Fig. 7B), which was found to be of the form:
I (A) = 1.094 × 10−11 + 3.6806 × 10−14exp(0.14336·RH(%)) | (1) |
The relative humidity sensor signal (I/I0) displayed an exponential current change with increasing RH% (Fig. 7C). Furthermore, the sensitivity of the sensor (S) was estimated as the first derivative of the relative sensor signal with respect to the concentration of the analyte, i.e. ambient RH% and is shown as the inset in Fig. 7C. Last but not least, the response and recovery time of the sensor was evaluated for the different contents of relative humidity.
In order to determine the recovery and response time of the sensors (Fig. 7D), the following process was followed:52 the experiment was started with the chamber being set at 0% RH% and a steady current value is measured for the given bias. When a new RH% pulse is introduced into the chamber, the current starts to increase until it reaches its new steady state value, which is proportional to the RH% content. This means that the measured current changes with time, until it reaches a certain value, and then it remains constant with time. The increase in the current between the baseline current (at 0% RH) and the new (steady-state) value is then known. The response time (t90) is defined as the time taken by the sensor to reach 90% of this current increase from the moment the new RH% pulse is introduced. Respectively, the recovery time (t10) is defined as the taken by the sensor to reach 90% of the current decrease, from the moment the RH% pulse ceases. Following the above described method, the measured response time (t90) of the sensor varied from 15 s to 335 s for humidity ranging from 2% to 65%, reaching 1300 s for 75% RH, while the recovery time (t10) ranged from 4 s to 90 s with increasing humidity contents, comparable to those of drop-cast CQDs on IDEs from the literature.53
Relative humidity measurements: the response of the fabricated sensors to relative humidity content was measured as follows: the sensors were connected to a Keithley 2400 source measure unit (SMU) and then placed in a Teflon® chamber. The relative humidity content was controlled by supplying a mixture of dry air and humid air at a total of 1000 sccm into the chamber. Dry air is compressed air-zero (UN1002, 2.2) supplied through a Brooks® 5800-S mass flow controller (MFC). Humid air was created by supplying zero air to a bubbler containing DI water, and controlled using a manual mass flow meter. The RH% and temperature within the chamber were monitored using a Hanna HI 9565 dewpoint hygrometer. A 2 V constant bias was supplied using the SMU and the current of the device was measured and recorded.
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