Widya Ernayati Kosimaningrum

Mekhaissia Ouis

Yaovi Holade
email: yaovi.holade@enscm.fr

Buchari Buchari

Indra Noviandri

Mostefa Kameche

Marc Cretin

Christophe Innocent

Platinum Nanoarrays Directly Grown onto a 3D-Carbon Felt Electrode as a Bifunctional Material for Garden Compost Microbial Fuel Cell

The practical implementation of air-breathing microbial fuel cell (MFC) is critically linked to the development of efficient materials as cathode and electrochemically active biofilm-based anode. In this contribution, we demonstrate the feasibility by decorating a 3D carbon felt electrode with Pt nanoarrays (CF@Pt) as a bifunctional electrode material as efficient garden compost bioanode and airbreathing cathode. Half-cell electrochemical characterizations reveal that the onset potential of the anode reaction negatively shifts of about 800 mV to reach -0.4 V vs Ag/AgCl after the growth of the biofilm onto CF@Pt. The investigation before and after the biofilm formation shows that Pt nanoarrays act as excellent electron relays, reducing significantly the charge transfer resistance. For MFC application (with a proton exchange membrane), the use of CF@Pt as bioanode' scaffold and CF@Pt air-cathode enables a drastically enhanced power density of P max = 292.3 mW m -2 , and a high short-circuit current density j sc = 1.9 A m -2 .

Cogeneration devices that, in addition to their primarily role, enable the realization of a secondary task have historically been a hot topic of research. [1][2][3][4][5][6][7] Specifically, microbial fuel cells (MFCs) that have emerged as a class of electrochemical devices with the ability of wastewater treatment, pollutant removal and concomitant net electrical energy harvesting from organic wastes are a typical example of cogeneration devices. 4,[8][9][10][11][12][13][14][15][16] Basically, the operation of MFCs needs electrochemically active biofilms that host certain bacteria for the organic matters degradation at an electrode so-called bioanode. Termed as cathode, the electron acceptor is typically an abiotic material, ranging from electroactive components such as platinum (Pt) for oxygen reduction reaction (ORR) to stainless steel plate dipped inside a ferricyanide solution. 11,[13][14][15][16][17][18][19] To date, two configurations in which electrodes face many limitations in terms of long-term operation and large-scale applications have been developed, double-chamber MFCs and single-chamber (or air-breathing) MFCs. 10,[20][21][22][23][24][25] Recently, Gaffney et al. 26 have reviewed the emerged computational approaches for understanding the microbial electrochemical systems in order to derive general designing principles. Some factors related to anode material such as biocompatibility, stability, surface area, conductivity, and charge transfer resistance should be considered to provide an optimal environment for the growth of the electroactive biofilm. Various types have been explored, especially carbon based materials such as carbon felt, carbon cloth, graphite felt, graphite rod, graphite granules and activated carbon. 11 In addition to its excellent biocompatibility, the use of carbon felt as anode material enables maximizing the amount of electroactive species owing to its tridimensional (3D) structure. However, the pristine material exhibits high hydrophobicity that could contribute to a relatively high resistance and low performance. In order to improve the bacterial adhesion and increase the extracellular electron transfer, various tactics have been adopted. 21,27 The incorporating of nanoscale materials seems promising to provide better interactions with biological systems due to the close dimension, special electronic properties, and parades as a good strategy to enhance the electrochemically active surface area and spread up the kinetics. [27][28][29] Combined with a flow and double-chamber MCF, the strategy improves significantly the performance, as noticed by Zhang et al. 27 They obtained a maximum power density of ∼2000 mW m -2 for a bioanode made of reduced graphene oxide and manganese nanocomposites on carbon felt, which was 154% higher than that with a bare anode. We have previously demonstrated that a surfactant-and binder-free hierarchical nanostructured Pt can provide best up performance and longevity as an air-breathing cathode. 21 To date, there are still key and unanswered questions about the electronic transfer capacity and the growth of electroactive biofilms when nanoparticles decorate the fibers of a 3D network. This kind of investigation is important to advance the field towards the achievement of high electrical performance with MFCs. In this work, we examine the design of a garden compost MFC system wherein metallic particles will simultaneously act as bridge between the biofilm and its electrode' scaffolds, and an electrocatalytic interface for ORR via air-breathing cathode. To date, this type of bifunctional materials composed of Pt nanoarrays directly grown onto an electroactivated carbon felt (CF@Pt) has not been reported for the targeted application as bioanode in MFCs. Complementary electroanalytical methods of cyclic voltammetry (CV), linear voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) were tightly combined to interrogate the processes taking place at the cathode and the anode.

Experimental

Chemicals and materials.-Three-dimensional carbon felt electrode (6.35 mm thick, 99%) was obtained from Alfa Aesar. Sulfuric acid (H 2 SO 4 , 97%), ethanol (VWR, 96%), hexahydrate hexachloroplatinic (IV) acid (H 2 PtCl 6 •6H 2 O, 99%), and potassium chloride (KCl, 99%) were purchased from Sigma-Aldrich. Sodium acetate (99%, Carlo Erba Reagents), and garden compost (Neuhaus) were used as received. A Milli-Q Millipore source was used to produce ultrapure water (18.2 MΩ cm at 20 °C).

Activation of 3D carbon felt electrodes.-The carbon felt electrode (CF) is hydrophobic so that tentative tests to deposit a significant amount of Pt particles failed. 21 So, CF was electrochemical activated prior to use. Basically, a sample was cut into a L-shape of 4 cm height, 1 cm width, and 0.635 cm thickness. Enough space was left on the upper side for electrical connection with a gold wire. By not taking into account the internal area, the estimated geometric area is 13.7 cm 2 . The sample underwent an ultrasonic cleaning step (water bath) with ethanol (2 h). It was then thoroughly washed with ultrapure water, and dried in an oven at 70 °C for 12 h. Then, the electrode was connected to as working electrode for activating in 1 M H 2 SO 4 by the method of CV. The counter electrode was a stainless steel plate. The reference electrode was a saturated silver/silver chloride (Ag/AgCl). The used potentiostat was SP-50 from Biologic Science Instruments and the utilized program was 30 cycles at 20 mV s -1 between 0 and 1.5 V vs Ag/AgCl. Finally, the electrode was copiously rinsed with ultrapure water until neutral pH before drying at 70 °C overnight.

Electrochemical growth of Pt nanoarrays onto 3D CF.-The preparation of Pt-modified CF followed the optimized conditions of our previous work. 21 Typically, the above three-electrode configuration was used. Electrodeposition was performed at -0.60 V vs Ag/AgCl (i.e., 0.14 V vs reversible hydrogen electrode (RHE) at pH 9) for 1 h under N 2 atmosphere. The potential was fixed higher than 0 V vs RHE in order to suppress immediately any formed H 2 during the process. The as-fabricated material was referred to as CF@Pt.

Characterization of the electrodes by scanning electron microscopy (SEM).-The materials that were not modified by the biofilm were directly analyzed by SEM on a Hitachi S-4800 microscope to gain textural and morphological insights. For the materials with biofilm, i.e., the bioanode samples, they were firstly pretreated to strengthen biofilm attachment and to remove water. Typically, a small piece of bioanode (0.5 × 0.5 × 0.635 cm) was immersed in 4 wt.% of a glutaraldehyde solution for 4 h. Then, the sample was carefully rinsed with ultrapure water. Dehydration process was carried out by the maceration of the sample in a series of graded concentration of ethanol solution (20, 40, 60, 80, and 98 wt.%) for 10 min in each concentration. Finally, the sample was dried in a desiccator at room temperature for 24 h before analysis by SEM.

Electrochemical characterization of the anode under MFC configuration.-Half-cell polarization of the anode at the different stages of biofilm growth was recorded in either a double-chamber MFC system or a single-chamber MFC one. Both operated under a switchable configuration of three or two electrodes thanks to the ON/ OFF mode of the reference electrode (RE). A Nafion 115 was used as the proton exchange membrane (PEM) and was activated overnight in water prior to use. For both configurations, the inoculum source was the garden compost leachate obtained by lixiviation. Garden compost and 60 mM potassium chloride solution (equivalent volume ratio of 1:1) were mixed and stirred for 24 h. 21,30 After the filtration on a sieve, a garden compost leachate was recovered followed by the addition of sodium acetate (final concentration = 20 mM). Double-chamber configuration utilizes a large surface area stainless steel plate as the counter electrode (in contact with 20 mM potassium ferricyanide solution), Ag/AgCl reference electrode, and scrutinizes 3 working electrodes (CF (20 cm 2 ), CF@Pt (14 cm 2 ), and bare graphite (GB, 13 cm 2 )). The current density was based on the estimated geometric surface area of each electrode. Singlechamber utilizes a CF@Pt anode connected to as working electrode, an Ag/AgCl reference electrode, and CF@Pt air-breathing cathode connected to as counter electrode. To prepare the latter, Nafion 115 PEM and CF@Pt electrode were pressed together to leave 3.14 cm 2 as exposed surface area. PEM orientated towards the anodic solution and CF@Pt is directly exposed to the surrounding air in the room. The methods of CV, LSV, and EIS were used for the electrochemical characterization.

Electrochemical characterization of the air-breathing cathode under MFC configuration.-For the half-cell polarizations of the air-breathing, the above conditions using single-chamber microbial fuel cell system were utilized to probe different materials (CF, and CF@Pt) connected to as working electrode. This air-breathing cathode was prepared by the above procedure to fix Nafion 115 PEM to CF or CF@Pt. The characterization methods were CV, LSV, and EIS. The current was normalized by the exposed surface area, which is 3.14 cm 2 .

MFC testing.-The content of each compartment is identical to the previous descriptions. To grow the biofilm, a resistance of R = 1 kΩ was connected to the MFC. The electrical characteristics of the MFC were recorded by utilizing the technique of "constant resistance discharge." Variable external resistances from 2 MΩ to 10 Ω were applied to the MFC; it is important to start by the high resistance where the cell voltage is roughly that of the open circuit voltage (OCV). After a few minutes of stabilization, the current and power were derived from the applied resistance and the measured cell voltage. Then, the current density (j, A m -2 ) and power density (P, W m -2 ) are obtained after normalizing by the area of the currentlimiting process, i.e., the cathode. The MFC was also characterized by EIS at OCV's conditions (see text).

Results and Discussion

Developed setup for microbial fuel cell scrutinizing.-Different cell and electrode configurations were designed to provide relevant information about the operation of each bio-electrocatalytic process. Figures 1a-1b sketch the designed setup including a reference electrode (RE) to probe the potential of each electrode during the fuel cell operation, and a connected resistance R = 1 kΩ to grow the biofilm. To have a better baseline performance, we run control experiments with other materials, as recommended by Logan et al. 11 The cathodic reaction occurring in a conventional double-chamber MFC (Fig. 1a) is the redox process Fe(CN) 4 3-+ 1e -→ Fe(CN) 4 4-. This means that the ferricyanide should be either continuously supplied or regenerated. The first scenario in practically unrealistic because it will require an excessively high amount of this harmful compound. The second option of regeneration could, in principle, work given that the redox couple Fe(CN) 4 3-/Fe(CN) 4 4-has fast kinetics on a large number of electrodes. However, this would mean that one has to stop the operation of the MFC for a certain time in order to make the regeneration. The other major issue about this regeneration option is that when the concentration of the ferricyanide falls below a certain limit, it will negatively impact the performance of the MFC. Then, the alternative is to use the unlimited amount of O 2 from the air as the electron acceptor. However, running that by a direct bubbling into a solution leads to low efficiency and a heavy system so that air-breathing cathode strategy (Fig. 1b) is the best option. Hence, we examined the 2-layered and 3-layered strategy for the biofilm fabricating (Fig. 1c), hypothesizing that the interacted platinum particles will provide positive effect by accelerating the electrons transfer between the bare electrode (CF) and the formed biofilm.

Physicochemical characterization of the electrodes.-The SEM images of such materials (GB, CF, and CF@Pt) before and after the biofilm formation are shown on Figs. 2a-2f. Additional SEM images of CF@Pt are reported in Fig. S1 (available online at stacks.iop.org/ JES/168/025501/mmedia). The used CF is made of fibers with a size of 15-20 μm. Furthermore, it can be observed that platinum particles with nanoscale domains (Fig. 2e) are well-dispersed on the carbon felt electrode. The particles size is, however, heterogeneous, from small nanoparticles of 10-50 nm to microparticles of 0.1-1 μm with a rough surface. Such heterogeneity could contribute to the formation and the anchoring/tethering of high amount of electrocatalytic biofilm. Figures 2b, 2d and2f show that the bacteria colonized the entire surface of the electrodes. Owing to its 3D structure and the presence of Pt particles, the biofilm modification of CF@Pt is expected to bring significant enhancement in terms of the bioelectrocatalytic activity. Electrochemical characterization in half-cell.-Having revealed the microscopic structure of these potential electrodes, we next sought to examine carefully and electrochemically their half-cell performance by using CV, and LSV as powerful tools. To mimic the real fuel cell operation, we designed an in situ garden compost MFC system in which CV and LSV are performed. Figures 3a-3c display the CVs recorded at 10 mV s -1 for the anode at the different stages of the biofilm formation using a double-chamber MFC. For the singlechamber, the results for CF@Pt and CF@Pt@biofilm are shown in Fig. 3d. The analysis demonstrates an increase in the oxidation current density upon the formation of the biofilm, when the starting material is either GB or CF (Figs. 3a, and3b). Unlike this observation, the CV profiles in Fig. 3c before and after the biofilm formation substantiate the coverage of CF@Pt by the biofilm because the signals related to protons adsorption and desorption disappear completely. The same observation can be made for the air-breathing cathode configuration in Fig. 3d. This suggests that a potential direct interaction of platinum particles with garden compost solution is inhibited. Given the absence of the characteristic proton adsorption-desorption signal that distinguishes platinum, it can consequently be deduced that the electrocatalytic activity of the CF@Pt@biofilm electrode will be uniquely characteristic of the bacterial culture.

To gain more insights on the MFC operation, the polarization curves of the anode before and after the biofilm formation were recorded by LSV at the low scan rate of 0.005 V s -1 . The data are shown in Fig. 4a for the double-chamber configuration and those of single-chamber are displayed in Fig. 4b (upper). For the first configuration, we firstly examined the metric of the open circuit potential (OCP) also termed to as E I=0 that theoretically indicates, under thermodynamic regime, the point at which the reaction should start for a rapid system. For all bare anode materials, OCP is +405, +285 and -15 mV vs Ag/AgCl for CF@Pt, GB, and CF respectively. These relatively high values are related to the dissolved O 2 in the beginning of the MFC operation, which is particularly true for any solution that is in contact with atmospheric air. 31,32 The high value of OCP = 0.4 V vs Ag/AgCl for CF@Pt confirms that the as-deposited platinum particles are electrocatalytically active to catalyze ORR. The standard redox potential of the couple Fe(CN) 4

3-/Fe(CN) 4 4-being ∼0.2 V vs Ag/AgCl, it means that without biofilm no significant cell voltage (E cell = E cathode -E anode ) can be extracted. After the biofilm formation (roughly three weeks later), anode's OCP shifts toward lower potentials to be -400, -175 and -390 mV vs Ag/AgCl for CF@Pt@biofilm, GB@biofilm, and CF@biofilm respectively. The results highlight an improvement of the OCP for 3D carbon felt electrode in comparison to the bare graphite plate with a downshift of 200 mV. While the GB@biofilm's OCP does not change significantly, it is worth of noting that the OCP of CF-based materials are remarkably approaching the theoretical value of sodium acetate oxidation, i.e. -496 mV vs Ag/AgCl. 31 The negative values of OCP suggest the electroactivity of the bioanode as well as the anaerobic condition. 31,33 OCP by itself is not sufficient to judge the ability of an electrode to drive proficiently an electrochemical reaction and deliver high currents under realistic conditions. So, by examining the current densities at an electrode potential E > OCP in Fig. 4a, the composite electrode referred to as CF@Pt@biofilm has fast a bio-electrocatalytic kinetic to achieve high current densities a low overpotential. For instance, 1 A•m -2 can be achieved for CF@Pt@biofilm at ca. 0.2 V vs Ag/AgCl whereas no other tested electrode allows that so far.

We subsequently turned out our switchable configuration of three/two electrodes design to examine the single-chamber MFC. The LSV profiles of the anode in Fig. 4b (upper side) clearly point out that CF@Pt@biofilm (OCP = -0.41 V vs Ag/AgCl) as bioanode outperforms CF@Pt (OCP = 0.14 V vs Ag/AgCl). It comes up that not only the biofilm formation is more facilitated at the surface of CF@Pt but also leads to an electrode with high current densities compared to its counterpart. For the cathode reaction of oxygen reduction (Fig. 4b, down side), the achieved OCP of 0.3 and 0.6 V vs Ag/AgCl for CF, and CF@Pt, respectively, demonstrates that Pt nanoarrays are still electrocatalytically active even if the anode compartment (garden compost + acetate + potassium chloride) contains highly poisoning species that could interferences. Kinetically, the LSV shows a significantly higher ORR's performance for CF@Pt air-breathing cathode due to the contribution of Pt active sites. From those set of results, it can be lastly concluded that CF@Pt could act as an excellent multifunctional material for use as both air-cathode for abiotic O 2 reduction into 4e -(previously verified 21 ) and interfacial substrate for efficient bacterial colonization for the bioelectrocatalytic organic matters oxidation. The Pt loading was determined to be lower than 0.2 mg cm -2 , which should justify the integration into practical air-breathing MCF as electrode material.

Microbial fuel cell testing.-We next performed the fuel cell tests to verify whether the observed trends in half-cell hold in real situation or not. For that, based on the previous findings, the electrical performance was firstly recorded for the double-chamber configuration for the three types of bioanode, i.e., GB@biofilm, CF@biofilm, and CF@Pt@biofilm. Figure 5a shows the obtained results. The OCV values are 0.49, 0.69, and 0.68 V for GB@biofilm, CF@biofilm, and CF@Pt@biofilm, respectively. These data confirm the previous evolution of the OCP. For the electrical metrics, the performance decreases in the order CF@Pt@biofilm (P max = 121.6 mW m -2 , j sc = 0.66 A m -2 ) > CF@biofilm (P max = 51.4 mW m -2 , j sc = 0.41 A m -2 ) > GB@biofilm (P max = 31.1 mW m -2 , j sc = 0.19 A m -2 ). In other words, P max was dramatically increased by 2.4 times after inserting Pt particles between CF and biofilm. Both CF@biofilm, and CF@Pt@biofilm outperform the GB@biofilm bioanode. Such findings in real fuel cell application are fully consistent with our hypotheses of increased performance upon the growth of platinum particles before the biofilm formation.

To extend the scope and get deeper insights, the best performing CF@Pt@biofilm bioanode was selected to be combined with CF@Pt air-cathode for single-chamber MFC testing. Of course, CF@biofilm was tested as the control bioanode to evidence any key contribution of the 3-layered structure. The results are presented in Fig. 5b. The recorded OCV is ca. 0.6 V for CF@Pt-based bioanode, which is substantially larger than the value achieved by the control material, CF. The fuel cell metrics of j sc and P max are 1.9 A m -2 and 0.3 W m -2 , respectively for CF@Pt-based bioanode. The recorded values of j sc = 1.1 A -2 and P max = 0.14 W m -2 for CF-based bioanode highlight an enhanced electrical performance in the presence of Pt nanoarrays. A sizeable increase in the performance of MFC due to metallic nano-species (Pt, Au, Mn, Fe, etc) has been widely reported. 11,15,17,21,27 It should be noted that the present electrical performance is still lower than the dreamed P max = 1-2 W m -2 when operating with double-chamber and/or continuous flow conditions but higher than most reported systems. 11,14,15,17,21,24,27,[34][35][36] To explain the high performance of the three-layer material of CF@Pt@biofilm, we hypothesize that the sandwiched Pt particles facilitate the electrons transfer more effectively between biofilm and electrode by stimulating the secretion of mediators. Some bacteria secrete mediating compounds such as phenazine and quinone, which are used to shuttle electron from the bacteria outer cell to the surface through mediated electron transfer mechanism. Hence, we further used electrochemical impedance spectroscopy to examine separately, the anode, the cathode, and the fuel cell. Figures 6a, and 6b depict the complexplane Nyquist plots. The profile of slightly depressed semicircles is characteristic of a charge transfer process with contribution of diffusion. This corresponds to an equivalent electrical circuit of R Ω + Q CPE //R ct (Fig. 6c) for the double-chamber MFC, and R Ω + Q CPE //(R ct + W) (Fig. 6d) for other processes in doublechamber and air-breathing MFC. R Ω is the uncompensated ohmic resistance, R ct is the charge transfer resistance, Q CPE is the constant phase element, and W is the Warburg element. [START_REF] Lasia | Electrochemical Impedance Spectroscopy and its Applications[END_REF][START_REF] Orazem | Electrochemical Impedance Spectroscopy[END_REF] The determined ohmic resistance is R Ω = 92 ± 4, 107 ± 3, and 221 ± 2 Ω for CF@Pt@biofilm, CF@Pt air-cathode, and fuel cell, respectively. Taking into account the exposed areas, R Ω = 1288, and 336 Ω cm 2 for CF@Pt@biofilm (exposed: 14 cm 2 ), and CF@Pt air-cathode (exposed: 3.14 cm 2 ), respectively. The high value of R Ω for the bioanode in comparison to the air-cathode results mainly from the formed biofilm (Figs. 2e vs 2f). R ct was evaluated to be 19 ± 2, 146 ± 1, and 153 ± 2 Ω for CF@Pt@biofilm, CF@Pt air-cathode, and fuel cell, respectively. The trend reveals that the kinetics is limited by the air-cathode. Note that with the surface area, R ct is 266, and 458 Ω cm 2 for CF@Pt@biofilm (exposed: 14 cm 2 ), and CF@Pt air-cathode (exposed: 3.14 cm 2 ), respectively. So the conclusion stands after the normalization. Basically, R ct is inversely proportional to the rate constant (k°), which means a lower kinetics for the air-cathode in comparison to the bioanode. This fully supports quantitatively and kinetically the previous choice to normalize the current density and power density by the area of the cathode. Comparatively, in double-chamber MFC system, R ct ∼10, 12000, and 12000 Ω for CF@Pt@biofilm, cathode, and cell, respectively. Thus, it indicates that the air-breathing MFC is more efficient, where the reaction at the cathode is nearly two order of magnitude faster than the doublechamber. We also observed a decrease of the ohmic resistance of the CF@Pt@biofilm bioanode for double-chamber configuration, which was expected given the low current, i.e., low amount of biofilm (Figs. 4 and5).

Conclusions

In summary, the study shows that the design of a single synthesis method to fabricate an efficient multifunctional material that has the ability to withstand the poisoning effects of garden compost microbial fuel cell (MFC) is doable. Specifically, a scalable electrochemical approach was tightly optimized to decorate carbon felt (CF) by platinum nanoarrays (CF@Pt). This CF@Pt simultaneously acts as an electron relay for bioanode and an abiotic electrocatalyst in doublechamber and single-chamber (air-breathing) MFC powered by the garden compost. Using batch conditions with a low metal loading, an efficient air-breathing MFC was constructed (based on a proton exchange membrane). It was able to operate tens of days. Having a large surface area and excellent electrical conductivity, this binder-free nanoscale electrode was found to effectively promote the growth of electrochemically active bacteria and facilitate the extracellular electron transfer from bacteria to their host. The electrical performance of the airbreathing MFC drastically increases in the order CF@biofilm (OCV = 0.47 V, P max = 138.8 mW m -2 , j sc = 1.1 A m -2 ) < CF@Pt@biofilm (OCV = 0.59 V, P max = 292.3 mW m -2 , j sc = 1.9 A m -2 ). The present insights could inspire further developments with other metallic nanostructured particles to improve the efficiency of MFCs. This research contributes towards the design of efficient bio-electrocatalysts for the electrochemical energy conversion devices.

Figure 1 .

 1 Figure 1. Designed systems for the microbial fuel cell (MFC) testing and scrutinizing. (a) Experimental set-up for half-cell electrochemical measurements for MFC in conventional double-chamber. (b) Experimental set-up for MFC in the configuration of air-breathing cathode. (c) The two approaches for fabricating an active bioanode: without platinum nanoarrays (left side) and with platinum nanoarrays (right side). A load of R = 1 kΩ was used for the biofilm growth when the reference electrode (RE) is switched OFF.

Figure 2 .

 2 Figure 2. SEM images of: (a) GB without biofilm, (b) GB with biofilm, (c) CF without biofilm, (d) CF with biofilm, (e) CF@Pt without biofilm, and (f) CF@Pt with biofilm.

Figure 3 .

 3 Figure 3. (a)-(c) CVs recorded at 10 mV s -1 for the anode at the different stages of the biofilm formation in a double-chamber MFC: (a) GB (dimensions: 4 × 1 × 0.5 cm: 12.5 cm 2 ), (b) CF (6 × 1 × 0.635 cm: 20.2 cm 2 ), and (c) CF@Pt (4 × 1 × 0.635 cm: 13.7 cm 2 ). (d) CV recorded at 10 mV s -1 at the different stages of the biofilm formation in a single-chamber MFC: CF@Pt anode (4 × 1 × 0.635 cm: 13.7 cm 2 ), and CF@Pt air-breathing cathode.

Figure 4 .

 4 Figure 4. Half-cell polarization curves recorded at 0.005 V s -1 in garden compost leachate + acetate. (a) Different anodes in a double-chamber MFC. (b) Airbreathing MFC: anode (up: j ⩾ 0), and cathode (down: j ⩽ 0).

Figure 5 .

 5 Figure 5. (a) Cell voltage E (opened circles: left y-axis), and power density P (full circles: right y-axis) of the electrical performance of a double-chamber MFC with the same cathode (stainless steel cathode), and different bioanodes. (b) Cell voltage E (opened circles: left y-axis), and Power density P (full circles: right y-axis) of the electrical performance of the single-chamber MFC utilizing CF@Pt air-cathode and different bioanodes. Data were iR-drop uncorrected.

Figure 6 .

 6 Figure 6. (a) Complex-plane Nyquist impedance plots for a double-chamber MFC at the open circuit conditions: stainless steel = cathode, and CF@Pt@biofilm = bioanode. (b) Complex-plane Nyquist impedance plots at the open circuit for a single-chamber MFC: CF@Pt = air-breathing cathode, and CF@Pt@biofilm = bioanode. (c) Equivalent electrical circuit of R Ω + Q CPE //R ct . (d) Equivalent electrical circuit of R Ω + Q CPE //(R ct + W). Experiments were performed during the growth of the biofilm (load of R = 1 kΩ) and before a fresh fuel is added at current minimum.

Acknowledgments

The authors acknowledge financial support from the Ministry of Research, Technology and Higher Education of the Republic of Indonesia (RISTEKDIKTI, Indonesia), Institut Europeén des Membranes (IEM), and Universitéde Montpellier (France).

ORCID

Yaovi Holade https://orcid.org/0000-0002-8806-568X


Cédric Masante

Martin Kah

Clément Hébert

Nicolas Rouger

Julien Pernot
email: julien.pernot@neel.cnrs.fr

Non volatile photo-switch using a diamond pn junction

Ultrawide bandgap semiconductor technologies offer potentially revolutionary advances in the rapidly developing areas of quantum communication, short wavelength optics, smart energy conversion and biomedical interfaces. These strongly demanding technologies can be partly constructed using conventional devices but new hybrid architectures are needed to overpass current performances and add functionalities. Here, we propose a new concept based on the specific properties of a diamond pn junction combined with both an electric and optical control of the depletion region. Using this junction as a gate in a junction field effect transistor, we report a proof of concept of a non volatile diamond photo-switch. A diamond pn junction made with nitrogen deep donors in the n?side is demonstrated to be optically activated thanks to visible light. The n-type diamond gate is almost devoid of free carriers in the dark and thus insulating. Illuminating the device renders the standard electrical gate control of the transistor efficient. Without illumination, the device is frozen, keeping a permanent memory of the current state. This new way of operating the device opens numerous possibilities to store and transfer information or energy with applications in the field of electrical aircraft or aerospace electronics, power electronics, bio-electronics and quantum communication.

I. INTRODUCTION

Because of their exceptional characteristics, ultrawide bandgap semiconductors (UWBGS) like diamond, aluminium nitride, boron nitride, gallium oxide are already in focus for producing devices beyond state-of-the-art semiconductor technologies 1 . For electronics, most researchers and engineers are still using concepts envisaged for silicon and wide bandgap semiconductors (like silicon carbide and gallium nitride) where much smaller band gap energies are at stake. The limits of this incremental approach for the fabrication of UWBGS-based devices are already well known and established. In this paper, we propose to go beyond current paradigms by putting forward the design of novel hybrid technologies (optical and electrical) based on unconventional concepts which exploit the huge energy scale of UWBGS.

Diamond's outstanding properties like wide bandgap, elevated carrier mobility, high breakdown field and huge thermal conductivity make it a unique material for high power and/or high temperature and/or radiation-resistant electronics. The long valley-polarization relaxation times of electrons make diamond a good candidate for use as valleytronics transistor 2,3 . Diamond takes also advantage of point defects, like nitrogen vacancy centres, for applications in quantum sensing 4 , quantum communication 5 or spin-based photonic quantum technologies 6 . Finally, its bio-compatibility offers the possibility of integrating diamond electronics into the human body [7][8][9][10][11][12][13] . To summarize, this material provides new opportunities to locally probe electric or magnetic signals, to reliably process data and to safely communicate them in harsh or living environments. Diamond devices capable of storing charges in a non volatile manner or addressing individual devices are needed to develop monolithic diamond-based systems. As examples, the charge accumulations could be used for power supply (local on-chip energy storage) or information storage with read and write access (non volatile memory) and individual on-chip addressing for application in neuronal multiplexers. Embedded on-chip devices coupled with an ultra high conversion efficiency of beta-voltaic cell using diamond pn junction 15 can be envisaged for aerospace or harsh environment applications.

In this paper, we report on a diamond-based non volatile photo-switch with a permanent off state having no loss. The concept is demonstrated by using monocristalline p-type diamond deposited by plasma enhanced chemical vapour deposition on a highly nitrogen-doped diamond substrate manufactured using high pressure and high temperature. In the first part, the concept will be described. Then, the active and freeze state of the pn junction will be established and the proof of concept of a non volatile photo-switch will be demonstrated thanks to an electro-optical control of a diamond junction field effect transistor. Then, we will discuss examples of application fields in which this concept can efficiently operate such as power electronics, quantum communication or bio-electronics.

II. NON VOLATILE DIAMOND-BASED PHOTO-SWITCH CONCEPT

For analogue or digital operations, the ubiquitous silicon-based field effect transistors work by using a gate bias voltage to deplete (OFF state) or populate (ON state) the channel. In the hybrid structure proposed here, the external illumination activates or freezes the device. In this way, the device can be driven by a combination of electrical and optical signals, providing a higher degree of freedom. In conventional semiconductors with shallow energy levels associated to donors or acceptors, doping level and carrier concentrations are almost equal at room temperature. The doped material is conducting. In nitrogen doped diamond, nitrogen impurities in substitutional sites create deep donor levels at 1.7 eV from the conduction band. Consequently, at room temperature, electrons remain bound to the dopants. Even at high doping levels (10 18 cm -3 to 10 19 cm -3 ) nitrogen doped (N-doped) diamond samples are completely insulating, and so cannot be electrically driven. This is the freeze state of the device. Thanks to external illumination, the absorption of photons promotes some of the bound electrons to the conduction band, increasing the free carrier concentration. The device thus becomes conducting and prone to be electrically driven. This is the active state of the device. The control of the ON and OFF states of the transistor is still ensured by a gate bias voltage. However, the switch from ON to OFF will only be achievable during the active state. This concept has to be distinguished from photo-conductive semiconductor switch devices 16 which have also been demonstrated to use N-doped diamonds 17 well. Optically generated carriers most participate in conduction which means these devices require a high power pulsed laser source to be efficient. For example, the device reported in 17 makes use of a pulsed laser with pulse power of about 6.7 kW (the average power is 25 mW). In this work, a bulk boron doped (p-type) diamond is used as the channel and the nitrogen doped diamond as the gate. The lower ionization energy of boron acceptor (0.38 eV) means it can conduct at room temperature in the dark while the gate requires external optical excitation to generate free electrons. Optically generated carriers do not significantly participate in the channel conduction. Only the n-type gate junction is optically controlled by the light source.

III. OPTICALLY CONTROLLED DIAMOND PN JUNCTION

A schematic description of the sample structure is shown in Figure 1a and top view images in Figure 1b and1c. A white LED driven by a tuneable current was used to optically excite the sample from the top surface. The average power density of the beam was measured with a photodiode detector, ranging from 0 to 11 mW.cm -2 . Part of the light was absorbed and reflected by the top metallic layer. The Figure 1d shows a room temperature current-voltage characteristic under various optical power varying between 0 and 11 mW.cm -2 . A diode characteristic was clearly measured under optical excitation while the dark current was below detection limit (< 10 -8 A.cm -2 ) in the whole voltage range. The threshold voltage was around 3 V, as expected for a diamond pn junction where the Fermi level lies close to the deep nitrogen donor level in n-side region. This clearly demonstrates that a nitrogen doped and boron doped diamond junction can be optically activated by continuous low power white light excitation. The LED does not emit in the UV range required for electron-hole generation in diamond (λ < 225 nm) 18 . Also, no intrinsic carriers were generated from the valence band to the conduction band. The linear relationship between the substrate conductivity and the irradiance (Figure 1d) is characteristic of a one photon impurity ionization process. In this experiment, the white light was found to be efficient to observe a photo-current because the transition from the nitrogen impurity level to the conduction band is efficient for wavelengths lower than 550 nm with absorption coefficients of the order of 10 to 100 cm -119 . The corresponding absorption depths were between 1 and 0.1 mm, from the diamond surface. As expected, the forward current was greatly limited by the thick, resistive n-type substrate with a current density in the µA.cm -2 range in the forward regime at room temperature (the current was normalized by the p-type layer surface area, delineated by the mesa etching). One can however extrapolate that the pn junction conductivity could be increased by orders of magnitude by (i) using a higher power continuous light sources up to 1000 W.cm -2 light output power than can be achieved with LEDs or lasers, (ii) depositing transparent top contacts or illuminating from the backside of the sample, (iii) using a <500 nm light source to maximize the optical absorption, (iv) using a thinner N-doped substrate and (vi) forming a proper ohmic contact on the N-doped diamond. However this simple setup provides a strong proof of concept of the nitrogen-doped diamond electrical activation under light exposure.

IV. HIGH BREAKDOWN CAPABILITY OF THE PN JUNCTION GATE

As an UWBGS, diamond is expected to be able to sustain an extreme electric field before reaching its breakdown. Hiraiwa et al. 20 estimated the diamond breakdown field for a doping level [N A ]= 2 × 10 17 cm -3 at 10 MV.cm -1 and Kamakura et al. [START_REF] Kamakura | IEEE International Electron Devices Meeting (IEDM)[END_REF] at 7 MV.cm -1 , compared to 0.2 to 0.6 MV.cm -1 for silicon. However experimental demonstrations are challenging in diamond devices at the moment due to the presence of a strong leakage current which causes an early soft break-down. To circumvent this difficulty in this original structure, we performed the measurement at -50 °C in order to reduce the temperature dependent leakage current. Since the substrate carriers were optically generated, and the substrate was still more resistive than the p-layer, no significant variation of the diode resistance was measured. In the dark, the reverse bias is held by the full thickness of the insulating substrate and the current is below the detection limit for a bias up to 1 kV. However when the diode was reverse biased under optical excitation, because the N-doped substrate is activated with a doping level much higher than the p-type layer, the Space Charge Region (SCR) was found to extend almost exclusively in the p-side. Almost all the potential drop is then sustained by the 400 nm boron doped layer. The Figure 1e shows the obtained IV curve, exhibiting a clear change of slope at V=-156 V attributed to the junction's breakdown. Due to the large substrate serial resistance, the breakdown characteristic did not exhibit the sharp increase of current with a power law, but was instead immediately limited by the substrate resistance and appears linear. A local breakdown, not occurring on the full surface of the diode, might explain the higher serial resistance measured after the breakdown compared to the forward regime. This is represented by an additional serial resistance R BD in Figure 1e. The maximum electric field at breakdown has been estimated to be 5.2 MV.cm -1 in a 1D punch-through configuration. This is a remarkably high value which is among the highest critical fields reported for diamond and much larger than other common wide band gap materials such as SiC and GaN (ideal breakdown around 2-3 MV.cm -1 ). Since no special care to reduce the 2D and 3D effects was implemented in the fabrication process, the real value is most probably higher due to the well known electric field crowding effect. The high breakdown voltage of diamond pn junction will ensure the fabrication of reliable and robust devices able to work in high voltage operations under illumination but even more importantly, it ensures a galvanometric insulation of the source to the gate of the junction field effect transistor described below without illumination.

V. ELECTRO-OPTICAL CONTROL OF A DIAMOND TRANSISTOR

A junction field effect transistor device was fabricated on the same sample to demonstrate the first non volatile photo-switch using diamond pn junction. The current flows through the bulk of the p-type layer between the drain and source under bias. Under optical illumination, the gate contact on the back side of the substrate controls the SCR extension in the channel. The drain to source distance is 10 µm and contacts have a 70 µm width. It is remarkable that room temperature transistor characteristics under an optical power of 11 mW/cm? were obtained as shown in Figure 2a and 2b. The typical ON-state linear regime, pinch-off and OFF-state are schematically drawn in Figure 2c, 2d, and 2e, respectively. The pinch-off is due to the extension of the SCR close to the drain contact, when the drain to gate bias approaches or is greater than the threshold bias (i.e. the drain to source bias V DS is around -30 V or more for V SUB = 0 V). Due to the low carrier density in the nitrogen doped substrate, the gate junction can be forward biased (i.e. V SUB -4 V) without any significant gate current. The SCR modulated by the gate bias can be fully removed to increase the channel conduction (0.25 mA/mm). Figure 2a shows that a gate to source bias V SUB = -10 V significantly increases the drain current. The gate current remains orders of magnitude lower, i.e. less than 2 × 10 -6 mA.mm -1 . No drain current modulation from the gate could be observed without optical illumination.

A measured commutation cycle at a fixed drain to source bias V DS = -40V (Pinch-off) is shown in Figure 2f, where the freezing of the device's electrostatic can be optically controlled.

The following paragraph describes each step represented in Figure 2f: (1), the substrate to source bias V Sub is set at 0 V with the light on, a bulk conduction channel between the drain and source is obtained (normally-on device). ( 2), the light is switched off, the free electron concentration in the N-doped substrate is extremely low (freeze state). In an ideal case without compensation from acceptor ions at room temperature, the free electron concentration is below 10 4 cm -3 . (3)

The gate substrate electric command was applied (V Sub = 40V ) to fully deplete the channel (OFF state). However, because the N-doped gate is devoid of free carriers in the dark, no electrostatic modulation can occur in the dark and the channel remains conductive even if the gate voltage corresponds to a OFF state. ( 4) The light is switched on, providing the optical signal activating the device (active state). The device was able to respond to the electrical gate command and switches to OFF state. The channel was fully depleted and the drain current I DS dropped below the detection limit. (5) The light was switched off (freeze state). (6) The gate electric command to switch the device in ON state was applied, V Sub = 0 V, but the channel remained insulating due to the absence of optical excitation. (7) The light was switched on (active state) and the device was able to switch to the ON state. This sequence was replicated several times as shown in Figure 3a to ensure the reliability of the measurements. Although the first cycle seems different, all the following cycles were found to exhibit the same characteristic. In order to check the non volatile character of this optically activated switch, the device was tested during a long time in the off and freeze state. Figure 3b and 3c shows the drain current versus time for V DS =-40 V at room temperature and 100 °C, respectively for long duration (2 days at room temperature and 140 minutes for 373 K). One can observe that the I ON /I OFF ratio is larger than 4 orders of magnitude during more than 48 hours at room temperature and larger than 3 orders of magnitude during more than 1 hour at 100 °C. This non-volatile photo-switch benefits from the unique properties of diamond such as its huge breakdown field and high temperature operation.

The large RC time constant and, more precisely, the huge resistance R caused by the high substrate's thickness and resistivity meant that the turn-on time was long but the modulation was clearly observed. Effects of traps most likely present at the substrate/epilayer and epilayer/oxide interfaces could not be distinguished. Reducing the turn-on time is a key factor in obtaining an efficient device, but as discussed previously, this could be decreased by orders of magnitude as it directly depends on the N-doped layer resistance. Additionally, in this kind of FET device the optical excitation only has to be present during the short time of the device switching. As well as the previously mentioned ways to increase this simple proof of concept performances, the use of a pulsed light source could provide a stronger optical excitation with limited total power usage.

VI. NON VOLATILE PHOTO-SWITCH FOR NEW APPLICATIONS

The next challenge to create new devices will be to grow nitride-based light emitting diodes (LED) directly on diamond transistor. Indeed, creating electrically driven visible or sub-band gap UV LEDs on top of the diamond transistors will enable high optical power density excitation to be attained. This would in turn enable the efficient control of the non volatile photo-switch, i.e. to read or write the transistor state (ON or OFF thanks to the FET gate) and store or reset the transistor state (freeze or activate thanks to the LED electrode). This new device paves the way for numerous practical applications where the superior properties of diamond semiconductors can be advantageously exploited. For embedded electronics, non volatile memories, i.e. the association of a switch with a capacitance for example, could be very useful to store and read information in a harsh environments (radiation resistant and high temperature) like in aerospace and aeronautic applications. For power electronics, the gate terminal of classical power transistors is affected by high dV /dt and Miller coupling, causing undesired turn-ON and possible catastrophic shoot-through. Thanks to the perfect galvanic insulation of the gate terminal without illuminating light, the Miller coupling between the Drain and the Gate is completely removed thus offering the strongest immunity to parasitic turn ON. For quantum communication, the LED will make the excitation of a nitrogen vacancy (NV) centre possible but more importantly the diamond FET will also permit control of electrostatics and a non-volatile storage of the NV charge state. Finally, we shall describe a bio-electronics applications in which the unique properties of diamond in the context of this concept are fully exploited. Diamond-based electronic is a perfect candidate for chronic biomedical implants since it provides solution for most of the scientific barriers that prevent most advanced technology to be transferred to clinical applications. Its chemical inertness combined with its carbon nature make it a biocompatible material compatible with all the standard medical imaging techniques including Magneto Resonance Imaging [START_REF] Hébert | [END_REF] . Furthermore, diamond electronic can be easily sterilized without damaging its performances. Diamond electrodes and transistors have thus already proved to be very interesting candidate combination for interaction with the body for both sensing and stimulation 12,13 . This is particularly true in the case of neural implants where bidirectional communication is required to develop neural prosthesis, help neural regeneration and gain further understanding of brain signal processing and dynamics. Future diamond embedded electronics have the potential to create implants with a high level of stability that do not require heavy hermetic insulation as is the case with silicon and other emerging polymer and flexible logic nitrogen content, as shown in the optical top view of the sample in Figure 1b. But, conversely, the p-type layer doping level was confirmed by capacitance-voltage measurements, not shown here, performed on metal oxide semiconductor capacitors test devices, fabricated on the same sample.

B. Fabrication of diamond transistor

To lower the energy barrier between the top metal contacts and the p-type epilayer, a 200 nm highly boron-doped ([B]> 4 × 10 20 cm -3 ) diamond layer was selectively grown using an aluminium mask. A mesa etching was then performed down to the substrate by reactive ion etching with an oxygen plasma. This enabled the electric separation of each test devices on the sample surface from each other. The top metallic contacts were formed by a Titanium/Platinum/Gold (30 nm/20 nm/30 nm) stack deposited by electron beam evaporation and annealed for 30 min at 500 °C in high vacuum condition (< 10 -7 mBar). An UV ozone treatment was performed on the surface before the deposition of 50 nm alumina layer by atomic layer deposition to passivate the whole surface, followed by post annealing in high vacuum condition (< 10 -7 mBar) at 500 °C for 1 hour 24 . The top drain and source contact consists of a Titanium/Platinum/Gold (30 nm/20 nm/30 nm) stack, deposited by electron beam evaporation on the 50 nm alumina layer. No lithography step has been performed after the passivation oxide deposition. Also the metal oxide metal stack thus formed is electrically broken at the time of the first measurement by applying a few volts bias voltage. The detailed process is given in Ref. 25 . The back contact between the substrate and the gold layer on the alumina plate was done thanks to silver paste.

C. Electrical characterization of the device

The PN diode electrical characterization was performed using a Keithley 2612. The JFET electrical characterization was performed using a Modulab Solartron including the low current module. The electrical contacts were obtained using probes moved by a piezoelectric system with a LINKAM system to control temperature under vacuum. The microscope white LED light beam was used to optically excite the sample from the top. The use of a nitrogen-doped substrate as n-type layer is convenient and greatly simplifies the fabrication process. However it also causes a very large serial resistance R Sub due to its thickness (0.5 mm) and large resistivity. Also the silver paste back contact is probably of poor quality. Capacitance measurements are prohibited by the low cut-off frequency induced by the serial resistance so we only relied on static electrical measurements. This simple design was however found to be well suited to demonstrate this proof of concept.

Figure 1 .

 1 Figure 1. Optically controlled diamond pn junction a Schematic representation of the device and the measurement setup during the pn diode electrical characterization. The p-type layer resistance is neglected in comparison with the substrate resistance. b Optical view of the sample. c Secondary electron microscope top view image of a singular test device (colours added). d Room temperature current-voltage characteristic of the drain-gate pn diode activated by a white light LED with various optical power ranging between 0 and 11 mW/cm?. The source is floating and the gate/substrate is grounded. The current is normalized by the p-type layer area delineated by mesa etching. e Current-voltage characteristic of the pn diode under light exposure at -50 °C.

Figure 2 .

 2 Figure 2. Non volatile photo-switch using a diamond junction field effect transistor. a Quasi-static room temperature transistor characteristic, drain current versus drain-source voltage, measured for a substrate to source bias V Sub varying from -10 V to +30 V with 10 V step and continuous 11 mW/cm? optical power density. b Drain current density as function of V Sub in the Pinch-off regime at V DS =-40 V. c Schematic cross section of the device in ON-state at low V DS (linear regime), d at high V DS (pinch-off regime) and e in OFF-state. f Measured switched drain current over one commutation cycle of the fabricated JFET at V DS = -40 V, as function of the light and substrate bias. The device is only able to commute under an optical signal.

Figure 3 .

 3 Figure 3. Temporal stability of the non-volatile photo-switch. a Drain current density versus time for V DS =-40 V at T=297 K for multiple commutation cycles alternating freeze/active and on/off states by using various substrate voltages and optical powers as described by the chronogram. Drain current versus time for V DS =-40 V b at T= 297 K and c T= 373 K , for long duration in order to illustrate the non volatile character of the off state in the freeze state.

T. T. Pham, M. Gutiérrez, C. Masante, N. Rouger, D. Eon, E. Gheeraert, D. Araujo, J. Pernot, Appl. Phys. Lett. 2018, 112, 102103.

C. Masante, N. Rouger, J. Pernot, J. Phys. D: Appl. Phys. 2021, 54, 233002.

ACKNOWLEDGMENTS

The authors are thankful to David Eon (Univ. Grenoble Alpes, CNRS, Institut Néel, France), Etienne Gheeraert (Univ. Grenoble Alpes, CNRS, Institut Néel, France), Juliette Letellier (Di-amFab, France), Khaled Driche (DiamFab, France), Hitoshi Umezawa (National Institute of Advanced Industrial Science and Technology, Japan) and Robert Nemanich (Arizona State University, USA) for fruitful discussions.

electronics 10 . The concept offered by the photo-switch using a diamond pn junction is one of the first diamond implantable logic electronics which can be combined with a diamond sensor to develop a low power diamond time division multiplexer. Each line of an array of solution gated field effect diamond transistor can be sequentially read as in the case of CCD sensor, allowing for the reduction of the cumbersome electric wires that dramatically increase the size of the implant 23 .

Additional the freezing state of the switch means low power consumption can be achieved which is a key parameter for chronic implants.

VII. CONCLUSION

We demonstrated a novel concept of device with a hybrid optical and electric gate control. The pn junction made of N-doped diamond can be optically controlled using a white light LED. A breakdown of -156 V at -50 °C has been measured, corresponding to a peak electric field estimated to be superior to 5.2 MV.cm -1 . This proof of concept of hybrid electrical and optical switching device consists of a diamond p-type channel JFET. A clear transistor effect was measured under light exposure (active state) while almost no current modulation was observed in the dark (freeze state). The robustness of the non volatile photo-switch has been demonstrated during two days at room temperature and more than 140 minutes at 100 °C The high turn-on and turn-off times are the consequence of the thick, resistive, substrate used as the gate, the low light power density and the opaque metallic top contacts. They could be drastically reduced by simple changes to design of the device, opening the route to numerous applications in the field of power electronics, bio-electronics and quantum devices.

VIII. EXPERIMENTAL SECTION A. Diamond pn junction structure deposition

The sample structure is depicted in Figure 1a. The p-type layer consists of a 400 nm borondoped ([N A -N D ]= 2 × 10 17 cm -3 ) diamond layer grown by microwave plasma assisted CVD, provided by DiamFab, on a 4 × 4 × 0.5 mm 3 Sumitomo High Pressure High Temperature (HPHT) Nitrogen-doped (n-type) diamond substrate, also referred as 1b diamond. Its nitrogen content is estimated to be in the order of 10 19 cm -3 , mostly in single substitutional sites. The precise doping level was not measured but the sample exhibited the characteristic yellow tint of a high