Photoluminescence Probing of Complex H2O Adsorption on InGaN

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Photoluminescence Probing of Complex HO Adsorption on InGaN/GaN Nanowires 2

Konrad Maier, Andreas Helwig, Gerhard Muller, Pascal Hille, Jörg Teubert, and Martin Eickhoff Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b03299 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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Photoluminescence Probing of Complex H2O Adsorption on InGaN/GaN Nanowires Konrad Maier*, Andreas Helwig*, Gerhard Müller*,**, Pascal Hille***, Jörg Teubert***, Martin Eickhoff***,**** * Airbus Group Innovations, D-81663 Munich, Germany, ** Munich University of Applied Sciences, Department of Applied Sciences and Mechatronics, D-80335 Munich, Germany; *** Institute of Experimental Physics I, Justus-Liebig-University Giessen, D-35392 Giessen, Germany; **** Institute of Solid State Physics, University of Bremen, D-28359 Bremen, Germany

KEYWORDS. III-nitride semiconductors, nanowires, photoluminescence, H2O adsorption, surface recombination, surface passivation, photoelectrochemical water splitting.

ACS Paragon Plus Environment

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ABSTRACT. We demonstrate that the complex adsorption behavior of H2O on InGaN/GaN nanowire arrays is directly revealed by their ambient-dependent photoluminescence properties. Under low-humidity, ambient-temperature, and low-excitation-light conditions, H2O adsorbates cause a quenching of the photoluminescence. In contrast, for high humidity levels, elevated temperature, and high excitation intensity, H2O adsorbates act as efficient photoluminescence enhancers. We show that this behavior, which can only be detected due to the low operation temperature of the InGaN/GaN nanowires, can be explained on the basis of single H2O adsorbates forming surface recombination centers and multiple H2O adsorbates forming surface passivation layers. Reversible creation of such passivation layers is induced by the photoelectrochemical splitting of adsorbed water molecules and by the interaction of reactive H3O+ and OH- ions with photo-activated InGaN surfaces. Due to electronic coupling of adsorbing molecules with photoactivated surfaces, InGaN/GaN nanowires act as sensitive nanooptical probes for the analysis of photo-electrochemical surface processes.


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1. Introduction InGaN/GaN nanowires exhibit efficient photoluminescence (PL) in the green-light range, which persists well beyond room temperature into the range of 200-300°C1,2. The PL intensity of InGaN/GaN nanowires (NWs) is sensitive to the presence of oxidizing and reducing gases in the ambient air. Previous work has shown that O2, NO2 and O3 adsorbed at InGaN surfaces form efficient recombination centers for photo-generated electron-hole pairs1–3 while sensitivity towards reducing gases, such as H2 and hydrocarbons, can be achieved when the InGaN surfaces are covered with thin layers of Pt1,2,4. Although the latter detection mechanism is similar to the case of catalytic-gate-covered field effect devices5,6, the photoactivated detection process allows operation at significantly lower transducer temperatures of 120°C and below, thus enabling the analysis of molecule-specific adsorption processes rather than just detecting the combustion of adsorbed species as it is the case for most metal-oxide gas sensors that are operated at temperatures above 300°C. As in almost any conceivable gas sensing application, H2O molecules are present in large and largely variable quantities in the ambient air, the analysis of H2O adsorption on InGaN surfaces and its concomitant effects on the InGaN luminescence is investigated in the present work. Due to the relatively strong interaction of water with semiconductor surfaces and its high boiling point, H2O adsorbates, in principle, are able to form multi-layer BET (BET: Brunauer, Emmet, Teller) adsorbates on sensor surfaces7–10. Here, we use the photo-activated interaction of InGaN/GaN nanowire arrays (NWA) as nanooptical probes to analyze the formation process of complex water adsorbates, in particular the


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transition from single to multiple water adsorbates, which is reflected by a transition from quenching to enhancing PL response. Such an analysis is only possible due to the low operation temperatures during photo-activated adsorbate detection thus paving the way towards an optical analysis of chemical processes occurring on the surface of optical nanoprobes.

2. InGaN nanowire arrays as nano-optical probes InGaN/GaN nanowire arrays (NWA) as optochemical transducers (schematically shown in Fig.1)

were

grown

by

plasma-assisted

molecular

beam

epitaxy

(PAMBE)

with

0001 orientation on (111) silicon substrates, exposing non-polar m-plane 0011 InGaN/GaN surfaces at their side walls11. The composition of the InGaN section was investigated both by XRD- and PL-measurements yielding an In-content of 15% and 27%, respectively. This discrepancy is typical for InGaN-material and results from the formation of In-rich clusters within the InGaN matrix material12,13. Such nanowires exhibit a temperature-stable photoluminescence (PL) in the green spectral range up to temperatures well beyond 200°C that decreases in intensity when oxidizing gases are present in the ambient11. Here we study the evolution of the PL intensity upon exposure to different humidity concentrations in ambient air. For measurements of the humidity response the InGaN/GaN NWAs were mounted into a gastight chamber that allows optical excitation and extraction of the PL light through sapphire windows during exposure to synthetic air with a controlled degree of humidity. As illustrated in Fig.1a the photoluminescence was excited by an UV-LED with a wavelength of 𝜆 ~ 365 nm, and a maximum optical power of Popt ~ 300 mW via a dichroic mirror system. The luminescence light was collected and coupled into a photomultiplier tube (PMT).


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photom ultiplier

dichro ic mirror

nanow ire transduce r

Figure 1. (a) Schematic of the optical setup featuring a dichroic mirror, an optochemical GaN/InGaN transducer, a 365 nm power LED and a photomultiplier tube; (b) Morphology of the InGaN/GaN NWAs as seen by scanning electron microscopy (SEM, top-view). The inset shows a cross sectional view through a single nanowire and crystal structure of the InGaN/GaN nanowires. While their growth axis is aligned along their polar [0001] axes, gas exposure occurs at their non-polar side walls. In order to avoid unwanted side reactions between the adsorbed H2O and any co-adsorbed O2, most measurement sequences were performed in a background of dry nitrogen (N2) with the humidity level being controlled by passing a second flow of dry N2 through a water-filled bubbler and by admixing it to the first flow of dry N2. The timing of the humidity pulses and the corresponding levels of relative humidity (rh) are indicated by the filled boxes in the lower parts of Figs.2 and 3. Due to instrumental limitations, the actually measured humidity pulses in the vicinity of the InGaN/GaN NWAs proved to be rounded-off versions of the input humidity pulses. Except for the relative humidity in the ambient air itself, the parameters most


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significantly influencing the H2O adsorption are the temperature T of the InGaN/GaN NWAs and the illumination light intensity. As a measure of the latter we use the optical power Popt in the following.

3. Results In a first series of measurements at low excitation-light intensity (Popt = 0.7 mW) the PL-intensity response to a series of increasingly higher humidity pulses was comparatively investigated at room temperature (T ~ 30°C) and at 120°C. In Fig.2 it is seen that for the lower temperature (black dataset) the PL intensity is quenched upon onset of each H2O exposure pulse and that this quenching effect persists over the entire pulse duration of 20 min. As indicated by the dotted blue line, the magnitude of this quenching response is roughly proportional to the applied humidity concentration. For an elevated temperature of 120°C (red dataset in Fig.2) a different behavior is observed: Upon onset of each exposure pulse a similarly large initial quenching response is found that rapidly decreases as the H2O exposure is continued. After 20 min of H2O exposure, the initial PL intensity is almost restored and even a rapid PL overshoot is observed when the humidity exposure is terminated. Similar to the quenching response this PL overshoot fades out as dry nitrogen conditions are maintained. This experiment indicates that, in principle, H2O molecules perform as surface recombination centers and cause quenching of the NWA PL intensity. However, upon thermal activation, H2O adsorbates reorganize into a state with less pronounced quenching or even enhancing properties.


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1.01

PL normalized [-]

120°C

1.00

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Popt = 0.7 mW

90

30°C 120°C

60 30 0

60

120

180

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Figure 2. Change of the normalized PL light intensity of InGaN NWAs in response to a series of humidity exposure pulses (filled boxes) applied in a background of dry N2: (black dataset) NWA temperature 30°C; (red dataset) NWA temperature 120°C. In both experiments the LED optical power was maintained at 0.7 mW. The dash-dotted blue line indicates the scaling of the PL quenching response at T = 30°C with the humidity concentration. The data in Fig.3 shows the results of the same experiment as carried out at a significantly higher excitation light intensity (Popt = 200 mW). Similar to the previous case, an initial quenching response is observed upon onset of each H2O exposure pulse. Its magnitude is significantly smaller than in the low-excitation case, as evidenced by comparison to the dash-dotted blue line taken over from Fig. 2. At 120°C the initial quenching response is hardly visible anymore and almost pure PL enhancing responses are observed. This behavior can be attributed to a drastically faster expiration of the quenching response (Q) at high temperature and high excitation density conditions, resulting in an almost pure enhancing response (E). This Q to E (Q-E) transition is evidenced by the red dataset in Fig.3 that represents the results at high temperatures and high excitation density. The magnitude of this enhancing


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response saturates at a level which is independent of the humidity exposure level as indicated by

PL normalized [-]

the dashed red line in Fig. 3.

120°C

1.01 1.00

30°C 0.99 0.98

rh [%]

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Popt = 200 mW

90

30°C 120°C

60 30 0

60

120

180

time [min]

240

300

Figure 3. Change of the normalized PL-light intensity in response to humidity exposure pulses (filled boxes) in a background of dry N2 at high excitation-light intensity (Popt = 200 mW): (black dataset) NWA temperature 30°C; (red dataset) NWA temperature 120°C. The dashed red line indicates the saturation level of PL enhancement at T = 120°C; the dash-dotted blue line is taken from Fig.2 and marks the magnitude of the quenching response at T = 30°C under lowtemperature and low-light conditions. Whereas both Figs. 2 and 3 illustrate the temperature dependence of the Q-E transition, the influence of the excitation light intensity at a constant humidity level of 50% and a temperature of ~30°C is shown in Fig.4a. These results demonstrate that the speed of the Q-E transition is strongly increased and that larger levels of PL enhancement are attained at high excitation-light intensities. Measurements of full PL spectra using a slightly modified setup showed that the variations of the PL-intensity are restricted to the band-gap emission range (between 2.0 eV and 2.7 eV) with the


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effect being slightly more pronounced on the low energy side of the emission peak which is in accordance with literature on the effect of air ambient on InGaN nanowires14. Another important aspect concerns the role of O2 when similar experiments are performed in a background of dry synthetic air (SA). Fig.4b shows that for the same settings of humidity, LEDlight intensity and NWA temperature, a more rapid and more complete Q-E transition is observed, indicating the catalyzing role of O2 in the Q-E transition.

200 mW

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Figure 4. (a) Normalized PL intensity response to a humidity exposure pulse (rh = 50%) applied in dry N2 ambient for different LED-light intensities; (b) Effect of background atmosphere on the PL response to a humidity-exposure pulse: (black dataset) background of dry N2; (red dataset) background of dry synthetic air (SA).


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4. Discussion The above data provide clear evidence that water can adsorb at InGaN surfaces in at least two different forms which can convert into each other under appropriate experimental conditions. These two adsorbates differ in their exerted impact on the PL response of InGaN nanowires and thus can be classified as PL quenchers and PL enhancers. In Table 1 we list those experimental conditions under which water adsorbates act as PL quenchers and PL enhancers as well as possible assignments to the molecular character of both adsorbate forms and the formation processes involved as outlined in the following discussion. Table 1: (left column) Experimental conditions under which adsorbed H2O molecules act as luminescence quenchers or enhancers; (right column) possible clues to the adsorbate forms and their associated formation processes. H2O as a PL quencher Low humidity

Single (isolated) H2O adsorbates

Low light intensity

Photo-generated electrons and holes not involved in adsorbate formation

Low temperature

No re-arrangement of chemical bonds during adsorbate formation

H2O as a PL enhancer High humidity

Multi-molecular, more complex adsorbates

High light intensity

Photo-generated electrons and holes involved in adsorbate formation

High temperature

Re-arrangement of chemical bonds during passivating adsorbate formation


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4.1 Rate equations for adsorption, desorption and associated PL changes Here we show that the observed key features of the PL response under humidity exposure can be explained by a two-state adsorption model. In the following, we assume that both quenching (Q) and enhancing (E) H2O adsorbates are formed according to Langmuir processes15 and compete with each other, with the enhancing adsorbates prevailing in high-temperature and high-intensity light conditions. This situation can be described by a set of coupled rate equations for the surface coverages of quenching and enhancing adsorbates, 𝜃' and 𝜃( , respectively: )*+ ),

)*< ),

= 𝑟/)0,' (𝑝456 , 𝑇) 1 − 𝜃' 𝑡 − 𝜃( 𝑡

− 𝑟);0,' (𝑇) 𝜃' 𝑡 .

= 𝑟/)0,( (𝑝456 , 𝑇, 𝑃>?, ) 1 − 𝜃' 𝑡 − 𝜃( 𝑡

− 𝑟);0,( (𝑇) 𝜃( 𝑡 .

(1a)

(1b)

Here, it is assumed that the two kinds of H2O adsorbates can independently form on InGaN surfaces as long as empty adsorption sites are available, i.e. 1 − 𝜃' 𝑡 − 𝜃( 𝑡

> 0. 𝑟/)0 and

𝑟);0 are the rate constants for adsorption and desorption of both kinds of adsorbates. In view of the experimental results presented above, these rate constants can be expressed as follows: 𝑟/)0,' 𝑝456 , 𝑇 = 𝑠' 𝑟456 𝑝456 , 𝑇 exp −

𝑟);0,' 𝑇 = 𝑟K,' exp −

(EFG,+ HI J

(FLG,+

(2a)

(2b)

HI J


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𝑟/)0,( 𝑝456 , 𝑇, 𝑃>?, = 𝑠( 𝑟456 𝑝456 , 𝑇

𝑟);0,( 𝑇 = 𝑟K,( exp −

MNOP MNOP,QER

S

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exp −

(EFG,
?, − 𝛽 𝑇 (𝑁‰/n64 )(𝑁‚n4 )

(4a)

Under steady state conditions this latter equation predicts that the areal densities of Ga-OH and N-H surface bonds should scale with the square root of the optical power:

𝑁‰/n64 = 𝑁‚n4 =

Š J ‹ J

𝑃>?, ,

(4b)

which agrees with the experimental results. The other relevant observation, namely, that the formation rate of enhancing adsorbates, 𝑟/)0,( , is thermally activated while their desorption rate, 𝑟);0,( , is not, is easily explained by the fact that surface atoms need to convert from low-energy sp2 to higher-energy sp3 electron configurations to allow enhancing adsorbates to be formed, while the converse is true for their desorption process. Finally, Fig.8 shows that in the process of surface hydroxylation only one out of two H2O molecules is consumed, leaving the second H2O molecule either free to desorb or to tie up with the surface N-H and Ga(In)-OH groups via hydrogen bonds, possibly forming the starting layer of a multi-layer Brunauer-Emmet-Teller (BET) adsorbate7. A limiting form of such a BET adsorbate is a macroscopic volume of water in contact with a “wet” oxidized III-N surface. Depending on the pH value of the liquid electrolyte, either H3O+ or OH- ions are available which may exchange protons with the surface Ga(In) OH groups, thus transforming them into Ga-OH2+


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or Ga-O- surface groups. The resulting change in surface charge accounts for the established pHsensitivity of III-N surfaces22–24.

5. Conclusions In conclusion we have shown that water vapor can form complex adsorbates on non-polar III-N surfaces. These adsorption processes can be directly observed as adsorbate-induced changes in the PL response of InGaN/GaN NWAs as an experimental probe. H2O adsorbates can perform both as recombination centers and as surface passivating species. While this first role is given by the H2O molecular characteristics themselves, i.e. the availability of lone pair electrons, the latter role appears to be enabled by the capability of the III-N adsorbents to support photoelectrochemical water splitting reactions. Once formed, H- and OH-terminated III-N surfaces form ideal starting layers for multi-layer BET adsorbates. Moreover, the surface terminating OHgroups also appear to be responsible for the observed and stable pH response of III-N NWAs in aqueous solutions. ASSOCIATED CONTENT Supporting Information: Description of the model of non-radiative surface recombination and PL response. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author (J.T.) E-mail: [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgements We acknowledge financial support within the LOEWE program of excellence of the Federal State of Hessen (project initiative STORE-E).

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Photoluminescence Probing of Complex H2O Adsorption on InGaN

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