Ferry Anggoro Ardy
Nugroho
*,
Robin
Eklund
,
Sara
Nilsson
and
Christoph
Langhammer
*
Department of Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden. E-mail: ferryn@chalmers.se; clangham@chalmers.se
First published on 22nd October 2018
We demonstrate the transfer of arrays of nanofabricated noble metal and alloy nanostructures obtained by high-temperature annealing on a flat parent support onto optical fibers, to create a hysteresis-free fiber optic nanoplasmonic hydrogen sensor. This work enables the integration of complex nanofabricated structures and their arrangements in tailored arrays with fiber optics to realize optical sensors, which will find application in a wide range of disciplines.
To enable such a breakthrough, finding new ways to grow or deposit complex nanostructures, tailored in terms of size, shape and composition, onto optical fibers is necessary. To this end, such structures are readily available via nanolithography-based fabrication techniques where methods like electron-beam lithography and colloidal lithography have been demonstrated to enable the crafting of myriads of (complex) nanostructures with excellent control of the aforementioned key parameters.20–23 However, these methods typically require flat supports and, consequently, they are incompatible with optical fibers.
In response, in this communication we present a solution for the deposition of nanofabricated noble metal alloy nanostructure arrays onto optical fibers by further developing a pattern transfer method recently reported by Lodewijks et al.24 Specifically, we transfer PdAu alloy nanodisk arrays, fabricated by Hole-Mask Colloidal Lithography (HCL) on a flat support25 compatible with high-temperature annealing at 500 °C, onto optical fibers. In this way, we enable a fiber optic nanoplasmonic hydrogen sensor that features hysteresis-free and fast response, for which the high-temperature treatment necessary to induce alloy formation can be carried out prior to the pattern transfer, thus entirely eliminating thermal damage to the fiber. Furthermore, the use of nanofabricated nanoparticle arrays with controlled composition provides access to design rules that enable the tailoring and maximizing of the sensor limit of detection by engineering nanoparticle size and shape and thus their plasmonic response,26 yielding performance superior to thin film solutions.27,28
We chose this particular application to demonstrate our nanopattern transfer method because a fiber optic platform is very attractive for hydrogen detection due to its effective remote readout that reduces the risk of spark generation at flammable hydrogen concentrations (i.e. 4–75 vol% H2), as well as due to its small geometrical footprint and mass-production potential. To this end, fiber optic hydrogen sensors employing diverse sensing mechanisms such as evanescent field intensity,29–31 reflection,32–35 fiber-grating,36–38 interferometry39,40 and SPR41–43 have been reported. The majority of these sensors employ Pd (and its alloys33,40,44) as the active material since it enables effective hydrogen detection with high selectivity under ambient conditions as only hydrogen can induce the phase transformation to a hydride phase that gives rise to the optical contrast used as the readout.45 This contrast is induced by the absorption of hydrogen in interstitial lattice positions in the Pd host, which both alters the electronic structure, and the volume due to lattice expansion.46,47 However, all of these sensors, with a few exceptions,40 employ thin Pd films as active material, and thus they are prone to durability issues due to cracking and peeling of the films. Moreover, readouts other than evanescent field and reflectivity change often require complicated fabrication and setups.4
Also, the use of neat Pd as transducer material as such has significant shortcomings. For example, due to an energy barrier created by lattice strain during phase transformation to the hydride, hysteresis between hydrogen absorption and desorption is observed48 and hampers sensor accuracy and dynamic range. The former is adversely affected since the response will depend on sensor history, and readout may thus be ambiguous.48,49 The latter is limited in the sense that Pd features a large response only in a very narrow hydrogen pressure range, i.e. around the phase transformation in the α + β phase coexistence region.48,49 Finally, it is also predicted that thin films are inferior as hydrogen sensor transducers since nanoparticles have the potential to provide faster response due to shorter diffusion paths.50 Therefore, realizing a nanoplasmonic fiber-optic hydrogen sensor, where nanoparticles with tunable sensitivity via shape and size engineering, as well as with tailored chemical composition,26 act as signal transducers, is highly desirable. Specifically, Pd-based coinage-metal alloys are attractive because they feature faster hysteresis-free response and higher resistance towards poisoning and deactivation by species like CO and NO2 compared to neat Pd.44,51–53
To achieve such an optical nanoplasmonic Pd-alloy based fiber-optic hydrogen sensor, we have developed the specific sequence of nanofabrication steps outlined in Fig. 1 to transfer an amorphous array of alloy nanostructures made by HCL22,25 on a flat support, to an optical fiber. The key component enabling this process is the presence of a sacrificial Cr-layer and a thin C transfer layer that provide the means for lifting-off and transferring the nanostructures from the flat to the fiber support. Hence, these two layers are first deposited onto the flat support (here borofloat glass) by electron beam evaporation of a 100 nm thick Cr-film, followed by 10 nm of C (Fig. 1a). In the next step, the nanoparticles are defined by HCL nanofabrication onto the Cr/C sandwich support, followed by the layer-by-layer deposition of Au and Pd through the mask to produce a square centimeter quasi-random array of nanodisks with 170 nm average diameter, and a thickness of 25 nm (Fig. S1†). After lifting-off the mask, we annealed the sample at 500 °C for 24 h under H2 flow to induce the formation of a homogeneous PdAu alloy comprised of 75 at% Pd and 25 at% Au, as predefined by the respective thickness of the evaporated layers.25 We chose this particular alloy composition as it enables the complete suppression of hysteresis between hydrogen absorption and desorption, which is critical in a hydrogen sensing application.28,52 Mechanistically, hysteresis is suppressed by pre-straining the Pd lattice by the Au (or other atoms with different atomic radius than Pd, e.g., Ag, Cu or Ni25,44,52–54) and the concurrent lowering of the critical temperature for the formation of the hydride phase. Furthermore, the high-temperature annealing step is also necessary to increase the mechanical stability of the C-layer, to enable the pattern transfer.24
Fig. 1 (a) The steps of the growth of the sacrificial Cr-layer and the C-pattern-transfer-layer, followed by the Hole-mask Colloidal Lithography (HCL)22,25 nanofabrication of the PdAu alloy nanodisk array. (b) The sequence of the pattern transfer steps to the optical fiber. Once the Cr-layer is removed by wet etching, the C-layer with the nanoparticles is detached but still resides on the substrate. The substrate can therefore be used to pick up the C-layer and move it into deionized water. Due to the hydrophobicity of the C-layer, it will then readily float up to the water–air interface, providing access for the optical fiber to pick it up and complete the transfer process. Once dried, oxygen plasma is utilized to remove the C-layer, leaving only the nanodisk array on the fiber. Note that the schematic is not drawn to scale. |
After completion of the nanofabrication and alloy formation, the pattern transfer to the optical fiber can be initiated and executed following the steps depicted in Fig. 1b (see also Fig. S3† for photographs). First, the Cr sacrificial layer is dissolved by immersing the sample in a Cr-etchant solution, in this way detaching the C-film with the alloy nanoparticle array from the glass substrate. Once the Cr-layer is completely removed (as indicated by a distinct color change of the sample, see Fig. S3†) the C film can be picked up by the glass substrate and moved into deionized water. Due to the hydrophobic nature of the C film, it readily floats at the water–air interface. This makes it possible in the next step to “pick it up” by careful immersion of the desired part of the optical fiber and in this way transfer the nanofabricated pattern onto it (or essentially any other support of interest24). In this process, the very thin, flexible, and yet mechanically stable C film support enables the conformal transfer of the pattern onto the entire fiber (i.e. around it), if the correct size of the transferred pattern is prepared. After drying, as the final step, an oxygen plasma treatment is utilized to remove the C film, leaving only the array of alloyed Pd75Au25 nanodisks that reside on the fiber.
To evaluate the hydrogen sensing function of optical fibers decorated with a nanofabricated Pd75Au25 alloy nanoparticle array, we used a commercial multimode fiber with 300 μm SiO2 core and hard fluoropolymer cladding, and we implemented the optical readout as a combination of evanescent field and reflection mode, as shown in Fig. 2a. Specifically, we transferred the Pd75Au25 alloy particles onto the end of a fiber, where we previously had removed the cladding (see Fig. S2† for fiber configuration and the Methods section in the ESI† for details). In this way, upon hydrogen sorption in the nanoparticles, the evanescent field of the light transported through the fiber will be modified via coupling to the localized surface plasmon resonance (LSPR) in the nanoparticles, whose permittivity changes proportionally to the hydrogen concentration in the environment.26,55,56 This, in turn, gives rise to a wavelength-dependent variation of the transmitted light intensity,26,55,57 which we pick up in reflection mode by growing a 300 nm thick Al mirror at the tip, as a means to reflect the light back into a spectrometer via a bifurcated fiber (Fig. 2a and S4†).
Fig. 2b shows an optical micrograph of such a fiber, decorated with the transferred C film and the Pd75Au25 alloy nanoparticle array, revealing the conformity of the coating. The same fiber is shown after the oxygen plasma treatment, which removes the C-layer and leaves behind only the Pd75Au25 alloy nanoparticle array (Fig. 2c), as confirmed by SEM (Fig. 2d). To this end, we note that the minor particle agglomeration seen in the SEM image will not affect the particle sensing functionality since a distinct LSPR scattering peak around 550 nm can resolved by dark-field scattering spectroscopy (Fig. 2e).
Having successfully transferred an array of Pd75Au25 alloy nanoparticles onto a fiber, we now turn to assessing its hydrogen sensing functionality. For this purpose we placed the sensor in a flow reactor, which enables controlled exposure of the sample to hydrogen pulses in Ar carrier gas at a constant temperature of 30 °C, with gradually increasing hydrogen partial pressure (Fig. 3a, see also the Methods section for experimental details). At the same time, we measured the broadband optical transmission through our fiber probe in the 400–1000 nm range by employing a self-referencing scheme, that is, by defining the transmitted intensity at each wavelength as the zero baseline at the beginning of each measurement (Fig. 3b). Except at the wavelengths close to the LSPR peak of the alloy around 550 nm (in excellent agreement with the dark-field scattering spectrum in Fig. 2e), a distinct step-wise change in the optical signal appears when hydrogen is introduced, and whose amplitude nicely scales with the H2 partial pressure. In very good agreement with our previous work on flat sensors using a traditional nanoplasmonic sensing configuration, the highest change in optical transmission contrast, ΔT, occurs at the inflection points of the LSPR peak.53,58,59 Hence, peak-tracking is not necessary and monochromatic readout using cheap components like LEDs and photodiodes is readily enabled. To this end, plotting the ΔT at the 650 nm inflection point as a function of the square root of the H2 partial pressure in the feed reveals a linear relation obeying Sievert's law for a hydrogen solid solution in the host (Fig. 3c). This feature is in very good agreement with our previous studies of PdAu alloy hydrogen sensors on traditional flat supports,26,53 confirming that after the pattern transfer all sensor and material-related functionalities are retained.
To further corroborate these results, we also performed measurements on two negative control samples: a bare optical fiber with the cladding removed, and a fiber onto which we transferred an array of pure Au nanodisks with similar dimensions as the PdAu alloy structures. Since Au does not absorb hydrogen itself but acts as sensitive probe for its environment via LSPR,10 it provides insights into whether either the fiber itself or, e.g., C residues from the pattern transfer are responsible for the observed response to hydrogen. The wavelength-resolved ΔT change upon a step-wise increase of the H2 partial pressure from 0 to 40 mbar (Fig. 3d) is shown in Fig. 3e for the bare fiber and in Fig. 3f for the Au nanoparticle control, respectively. Clearly there is no discernible response in both control measurements, as further highlighted in Fig. 3g.
As the final step of our study, we first characterized the hydrogen sorption properties, from a materials science perspective, of the transferred PdAu alloy nanoparticle array, as well as of a pure Pd analogue for comparison. Specifically, we measured the optical pressure-composition isotherms of both systems directly on the fibers at 30 °C, using the monochromatic ΔT at 650 nm as the readout (Fig. 4a). For the neat Pd, an isotherm characterized by the wide α + β phase coexistence region (“plateau”) reveals itself, together with the expected hysteresis between the hydrogen absorption and desorption branches.48 In contrast, the isotherm measured with the fiber decorated with the Pd75Au25 alloy nanoparticle array exhibits the anticipated complete suppression of the hysteresis44,53 due to the reduced critical temperature of the phase transition by Au-atom induced lattice strain,60 corroborating the homogeneous alloy formation.25,53 In this way, the sensing characteristics are greatly improved since accurate hydrogen detection with a wide dynamic range becomes available.53
We also investigated the temporal response characteristics of the Pd and Pd75Au25 fiber-optic sensors upon hydrogen absorption to and desorption from 40 mbar hydrogen partial pressure (i.e. lower flammability limit), respectively, at 30 °C (Fig. 4b and c). Clearly, the Pd75Au25 sensor outperforms the neat Pd in both response (t90) and recovery time (t10). This general trend is in good agreement with previous reports of these two systems at the qualitative level (note also that the faster response compared to the data in Fig. 3 is due to a larger applied H2 partial pressure difference). However, quantitatively, the response from both sensors is significantly slower than previously reported, where e.g. for Pd75Au25 a response time of <5 s was achieved.53 To explain this difference, we note that the experiment here was performed in a flow reactor with large volume61 and rather gradual partial pressure change (see Methods), in contrast to our previous study, which was done in a vacuum chamber that enables hydrogen pressure change at the sub-second timescale level.
Finally, we scrutinized the robustness of the transferred nanoparticles by exposing the Pd75Au25 fiber sensor to 110 cycles of 25% H2 in synthetic air carrier gas, to mimic real application conditions. As shown in Fig. 5, even with oxygen in the feed, the sensor responds consistently and with the same amplitude to hydrogen during the entire ∼20 h test. The slight change of baseline level that occurs at ca. 700 min we assign to a minute movement of the fiber. Since the sensor fiber is only loosely “clamped” into the bifurcator even the tiniest movement will slightly alter the amount of transmitted light and thus cause this artefact in our setup (see Methods). Nevertheless, the most important result here is that, despite the baseline shift, the overall response magnitude remains constant throughout the experiment in air.
In a wider perspective, our work opens the door to the integration of complex nanofabricated structures and their arrangements in tailored arrays with fiber optics to realize optical sensors, which we predict to find application in a wide range of disciplines, spanning from gas- and chemosensing to biosensing. The particular generic advantage that becomes available by our approach is that their sensitivity and optical fingerprint can be engineered and maximized by employing tailored nanostructures in terms of size, shape, arrangement and chemical composition,26 which are readily available by nanofabrication tools that are only compatible with flat surfaces.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nr03751e |
This journal is © The Royal Society of Chemistry 2018 |