EP3353801B1 - High-efficiency nanodiamond-based ultraviolet photocathodes - Google Patents

Description

• This invention relates in general to photocathodes for ultraviolet (UV).
• Caesium iodide (CsI), which has a band gap (6.2 eV) corresponding to the energy of UV photons is currently the most commonly used material for such photocathodes because it offers high efficiency. At the same time it has very little stability if exposed to air (because of its highly hygroscopic nature) or high photon flows (which dissociate the CsI, causing loss of iodine and oxidation of the caesium). Conversely, because of its band gap (5.5 eV), which is comparable to that of CsI, diamond is of great interest because of its very low electronic affinity, its chemical stability, its resistance to radiation and finally and not least because of its heat dissipation properties. These properties render diamond films optimum candidates for UV sensors. For these reasons poly- and nano-crystalline diamond (PCD and NCD) films have been used to produce UV photocathodes [1-8] having greater stability than that found in conventional CsI photocathodes [3]. In addition to this, NCD diamond films have been used to generate photocurrents and thermoionic currents at different temperatures [9].
• Normally diamond is used in the form of films deposited by means of chemical vapour deposition (CVD) techniques, obtained at temperatures around 800°C. Also it is well known that the polycrystalline diamond films obtained using the MWPECVD (MicroWave Plasma Enhanced Chemical Vapour Deposition) technique have a hydrogenated surface as soon as they are deposited, which imparts negative electronic affinity (NEA). This property is of crucial importance for the application of diamond in the construction of UV photocathodes, as it makes it possible to achieve the maximum quantum efficiency (12% at 140 nm) [1] for this material known to us in the international state of the art. This efficiency however does not remain stable over time and falls to 5% after approximately 1000 hours (45 days) ageing through mere exposure to air. The authors in reference 1 found that by hydrogenating the surface of the photocathode in hydrogen plasma its response was fully restored, and it again diminished if again exposed to air.
• In addition to what has already been said concerning stability, it is possible to add that further disadvantages of diamond films prepared using the MWPECVD technique are: i) the high temperatures involved, which limit their use to a restricted number of materials which can be used as a substrate, ii) the small dimensions of the substrate onto which the diamond films are deposited, iii) the high costs of MWPECVD technology for applications to large areas.
• One aim of this invention is therefore that of providing a photocathode for UV which is more stable and efficient than photocathodes based on CVD diamonds.
• Another aim of the invention is that of providing a process for producing a photocathode for UV in which the diamond layer can be deposited on the corresponding support at lower temperatures than in CVD methods.
• A further aim of the invention is to provide a process for producing a photocathode for UV in which the diamond layer can be deposited over more extensive surfaces at a lower cost than CVD methods.
• In view of these aims, the object of the invention is a process for producing a photocathode for ultraviolet, comprising
• providing a support capable of conducting electrons, and
• producing a photosensitive diamond layer on the support,
• in which production of the photosensitive layer comprises
• providing nanodiamond particles in powder form,
• hydrogenating the nanoparticles in an H₂ plasma,
• preparing a dispersion of the hydrogenated nanoparticles in a solvent, and
• spraying the dispersion onto the support and waiting for evaporation of the solvent from the support, the spraying and waiting cycle being repeated several times in order to obtain a continuous photosensitive layer.
• The abovementioned objects can be accomplished through the process according to the invention. In particular, photocathodes produced using the process according to the invention have the following advantages:
• simplicity of the production process and ease of deposition;
• high reproducibility;
• low production cost;
• possibility of deposition onto cheap supports of a disposable nature, including polymers (e.g. Kapton®) and paper;
• the possibility of restoring the efficiency of photocathodes on supports resistant to high temperature (e.g. doped silicon of the p, p-Si type) through post-hydrogenation;
• the possibility of reusing the nanodiamond layer (recycling of the material);
• high quantum efficiency;
• greater stability of quantum efficiency over time; and
• the possibility of incorporation into more complex deposition equipment which can incorporate more energy harvesting effects (thermoelectric, betavoltaic, photovoltaic, thennoionic, etc., effects).
• In addition to this, all stages in the production process are suitable for scaling up industrially, in that tools available on the market can be used together with nanodiamond particles; in particular spray systems suitable for treating large areas, using batteries of sprays in parallel, are already available.
• Preferred embodiments of the invention are described in the dependent claims, which are to be understood to be an integral part of this description.
• Further characteristics and advantages of the process according to the invention will become more apparent from the following detailed description of an embodiment of the invention with reference to the appended drawings provided purely by way of illustration and without limitation, in which
• Figures 1 and 2 are schematical illustrations of two embodiments of a photocathode according to the invention, in which the production of photocurrent stimulated by UV rays is also shown;
• Figure 3 is a graph showing absolute quantum efficiency (QE) as a function of wavelength for different photocathodes;
• Figure 4 is a graph showing the fall in absolute quantum efficiency over time through exposure to air for different photocathodes; and
• Figure 5 is a typical Raman spectrum of a nanodiamond particle layer.
• In order to produce a photocathode for ultraviolet the process according to the invention provides for the use of nanodiamond particles (hereinafter also referred to as "nanoparticles of diamond", or also merely "nanoparticles") in powder form. In addition to sp³ carbon (typical of the diamond phase), this powder also contains sp² carbon (typical of the graphite phase). The percentage of the sp² component with respect to sp³ may vary within a range from 70 to 87% as estimated by means of formula [10] $${\mathrm{P}}_{\mathrm{sp}2}=100\cdot {\mathrm{A}}_{\mathrm{sp}2}/\left({\mathrm{A}}_{\mathrm{sp}3}+{\mathrm{A}}_{\mathrm{sp}2}\right),$$

  

  where A_sp3 and A_sp2 are respectively the areas of the signals for the diamond phase (peak at approximately 1332 cm^-1) and the graphite phase (G-band) at approximately 1580 cm^-1 measured in the Raman spectrum (Figure 5) of the layer of nanodiamond particles. In particular, powders marketed by Diamonds & Tools sr1, Italy, were used for the tests described below. In these powders the average particle size is 250 nm.
• Particles of the powder indicated above are hydrogenated in a hydrogen (H₂) microwave plasma for a period of time of between 30 minutes and 3 hours at a temperature of between 850 and 1200°C. Because the material which has to be hydrogenated comprises particles, it is placed in a shallow container of material resistant to high temperatures, such as tungsten, and arranged in such a way as to maximise the exposed surface area of the particles. In addition to this, during the hydrogenation process the dimensions of the plasma are arranged to be slightly greater than those of the container so that the latter is wholly immersed within the plasma.
• A dispersion of the hydrogenated nanoparticles is then prepared in a solvent, for example a non-polar solvent such as 1,2-dichloroethane (DCE). Other solvents are however possible. Water, for example, can be used as a solvent. Dispersions may be obtained using standard ultrasound and centrifuging procedures.
• A support for the photocathode, which is capable of conducting electrons, is prepared separately. This support may comprise a substrate of conducting material, indicated by 10 in Figure 1. As an alternative the support may comprise a substrate of insulating material, indicated by 20 in Figure 2, on which a layer of conductive material 21, for example metal, is placed. p-type silicon (p-Si) substrates of thickness 500 µm were used for the tests described below in the first case, and Kapton® (thickness 50 µm + Al 20 µm) in the second case.
• The dispersion was then sprayed onto the support using the pulsed spray technique [11-14], using an ultrasonic atomiser for the spraying process and a heater on which the support which was to be coated was placed. This system was interfaced with a personal computer to control the parameters and automate the process. In order to produce a continuous uniform layer of nanodiamonds the spray pulses have to be repeated several times, separated by a waiting time which is necessary for evaporation of the solvent. Spray pulses lasting 15 ms were used for the tests described below, while the time between two pulses was 2 s. The pulse and waiting cycle was repeated 400 times. To encourage evaporation of the solvent the substrate was heated to a temperature of 120°C, a temperature higher than the boiling point (84°C) of DCE, during the spray and waiting cycle.
• In Figures 1 and 2 the photosensitive layer based on nanodiamond particles is indicated by 30, while the effective exposed surface area of the particles is indicated by 31. Figures 1 and 2 also show the surface hydrogen, indicated by 33. UV rays are indicated by 40, while the electrons emitted under UV irradiation are indicated by 50.
• With regard to the process of hydrogenation on the MWPECVD diamond film, which is carried out after the film has been produced, in the case of the procedure described above the hydrogenation process is carried out before the spraying process, and therefore before formation of the final layer. In addition to this the hydrogenation process should not be confused with various processes of annealing in hydrogen or oxygen known in the state of the art, as the purpose of the latter is to charge the surface of the nanodiamonds (of dimensions 4-5 nm) used to increase seeding, and therefore nucleation on substrates on which diamond films obtained using the CVD technique will grow, positively or negatively. In particular annealing in hydrogen is a process providing for the flow of a gas (molecular hydrogen H₂, which remains such) in a vacuum chamber in which the sample is held at a temperature of 500°C, and is therefore different from hydrogenation based on the interaction of an H₂ plasma with the diamond powder. In a plasma the H₂ molecule is dissociated into highly reactive H atoms, is ionised to produce ions of the H+, H₂+, etc., type, and is excited to form species of the H₂ ^*, H^* (Hα, Hβ, Hγ, etc.) type, which in decaying to their fundamental state (H^* → H+hv) produce the typical glow of a plasma. Under these conditions the production of active species is more efficient. These highly reactive species result in the formation of C-H bonds with very much higher probability than an annealing process in a flow of H₂.
• Various photocathodes were tested under UV radiation in a vacuum chamber. The quantum efficiency (QE) values of these photocathodes were calculated using that of a standard silicon calibration photocathode (NIST, National Institute of Standards and Technology). Figure 3 compares QE for a reference photocathode obtained using the MWPECVD technology described in reference 1 and five photocathodes produced using layers of nanodiamond particles:
1. (i) photocathode produced on a p-Si substrate using a process similar to that described above, but without hydrogenation of the nanoparticles;
2. (ii) prototypes A and B produced at different times (to test their reproducibility) on p-Si substrates using the process according to the invention described above; prototype C produced on a Kapton® substrate using the process according to the invention described above; and
3. (iii) photocathode produced on a p-Si substrate using a process similar to that described above, but with hydrogenation of the nanodiamond layer being carried out after deposition onto the p-Si.
• From Figure 3 it will be seen that the quantum efficiency of reference photocathode [1] is higher than that of photocathode (i), but lower than that of photocathodes (ii) and (iii). The three prototypes A, B and C (ii) exhibit maximum quantum efficiency of between 20 and 22% at 146 nm. The effect of hydrogenating the particles, in terms of both the gain in QE in the spectrum investigated and its increase at greater wavelengths, as a clear effect of lowering the electronic affinity of the material, can clearly be seen here.
  lowering the electronic affinity of the material, can clearly be seen here.
• Figure 4 shows absolute QE for five photocathodes (i), A, B and C (ii) and (iii) at 146 nm and reference photocathode [1] at 140 nm as a function of exposure time to air. It should be noted that the efficiency of the three prototypes (ii) relates to the wavelength of 146 nm and not 140 nm at which the QE would be greater than 20-22%, as can be extrapolated from the values in Figure 3. For prototype A (ii) the fall in QE after 51 days is 23%, for prototype B (ii) the fall in QE after 150 days is 25%, for prototype C (ii) the fall in QE after 5 days is 5%, while for reference photocathode [1] the fall is 58% after only 45 days, that is more than twice that for prototypes A and B.
References
1. [1] A. Laikhtman, Y. Avigal, R. Kalish, A. Breskin, R. Chechik, E. Shefer, Y. Lifshitz, "Surface quality and composition dependence of absolute quantum photoyield of CVD diamond films", Diamond Relat. Mater. 8 (1999) 725-73.
2. [2] J.S. Foord, C.H. Lau, M. Hiramatsu, A. Bennett, R.B. Jackman, "Influence of material properties on the performance of diamond photocathodes," Diamond Relat. Mater. 11 (2002) 437-441.
3. [3] M.A. Nitti, E. Nappi, A. Valentini, F. Bénédic, P. Bruno, G. Cicala, "Progress in the production of CsI and diamond thin film photocathodes", Nucl. Instrum. Methods Phys. Res. A 553 (2005) 157-164.
4. [4] A.S. Tremsin and O.H.W. Siegmund, "UV photoemission efficiency of polycrystalline CVD diamond films", Diamond Relat. Mater. 14 (2005) 48-53.
5. [5] M.A. Nitti, M. Colasuonno, E. Nappi, A. Valentini, E. Fanizza, F. Bénédic, G. Cicala, E. Milani, G. Prestopino, "Performance analysis of poly-, nano- and single-crystalline diamond-based photocathodes", Nucl. Instrum. Methods Phys. Res. A 595 (2008) 131-135.
6. [6] G. Cicala, M.A. Nitti, A. Tinti, A. Valentini, A. Romeo, R. Brescia, P. Spinelli, M. Capitelli, "Effect of properties of MWPECVD polycrystalline diamond films on photoemissive response", Diamond Relat. Mater. 20 (2011) 1199-1203.
7. [7] T. Sun, F. A. M. Koeck, P. B. Stepanov , R. J. Nemanich, "Interface and interlayer barrier effects on photo-induced electron emission from low work function diamond films," Diamond Relat. Mater. 44 (2014) 123-128.
8. [8] L. Velardi, A. Massaro, G.S. Senesi, M.A. Nitti, G. De Pascali, D. Melisi, M. Valentini, A. Valentini, G. Cicala, "Comparative photoemission study between nanocrystalline diamond films and nanodiamond layers", VI Workshop Plasmi Sorgenti Biofisica e Applicazioni - 2014, (2015) 77-81, ISBN: 978-88-8305-107-4.
9. [9] G. Cicala, V. Magaletti, A. Valentini, M. A. Nitti, A. Bellucci, D.M. Trucchi, "Photo-and Thermoionic emission of MWPECVD nanocrystalline diamond films", Appl. Surf. Sci. 320 (2014) 798-803.
10. [10] M.A. Nitti, G. Cicala, R. Brescia, A. Romeo, J.B. Guion, G. Perna, V. Capozzi, "Mechanical properties of MWPECVD diamond coatings on Si substrate via nanoindentation", Diamond Relat. Mater. 20 (2011) 221-226.
11. [11] Melisi, D.; Nitti, M. A.; Valentini, M.; Valentini, A.; Ditaranto, N.; Cioffi, N.; Di Franco, C., "Radiation detectors based on Multiwall Carbon Nanotubes deposited by a spray technique", Thin Solid Films 543 (2013) 19-22.
12. [12] G. Cicala, A. Massaro, L. Velardi, G. S. Senesi and A. Valentini, "Self-Assembled Pillar-Like Structures in Nanodiamond Layers by Pulsed Spray Technique," ACS Appl. Mater. Interfaces 6 (2014) 21101-21109.
13. [13] G. Cicala, G. Perna, D. Marzulli, D. Melisi, G. De Pascali, A. Valentini, G. S. Senesi, A. Massaro, L. Velardi, V. Capozzi, "Self-Assembled Structures in Nanodiamond Layer by Spray Technique", XII International Conference on Nanostructured Materials (NANO 2014), July 13-18, 2014 Moscow, Russia.
14. [14] A. Valentini, D. Melisi, M. A. Nitti, M. Valentini, L. Velardi, G.S. Senesi, A. Massaro, G. Cicala, "Photoemission response of spray deposited diamond layers at low temperature", International Conference on Diamond and Carbon Materials, 7-11 September 2014, Melia Castilla, Madrid, Spain.

Claims (7)

1. A method for producing an ultraviolet photocathode, comprising
   providing a support (10; 20, 21) capable of conducting electrons, and
   producing a photosensitive layer of nanodiamond particles (30) on the support (10; 20, 21),
   wherein producing the photosensitive layer comprises
   providing nanodiamond particles in form of a powder,
   hydrogenating the particles in an H₂ plasma,
   preparing a dispersion of the hydrogenated particles in a solvent, and
   spraying the dispersion onto the support and waiting for the solvent to evaporate from the support, this spraying and waiting cycle being repeated several times to obtain a continuous photosensitive layer.
2. A method according to claim 1, in which the nanodiamond powder contains sp³ and sp² hybridized carbon, where the percentage P_sp2 of sp² carbon varies in the range 70% ≤ P_sp2 ≤ 87%, as estimated from formula $${\mathrm{P}}_{\mathrm{sp}2}=100\cdot {\mathrm{A}}_{\mathrm{sp}2}/\left({\mathrm{A}}_{\mathrm{sp}3}+{\mathrm{A}}_{\mathrm{sp}2}\right),$$

   

   with A_sp3 and A_sp2 being the respective areas of the signals for diamond, or sp³ carbon, and graphite, or sp² carbon, as measured in the Raman spectrum of the diamond layer.
3. A method according to claim 1 or 2, in which hydrogenation comprises subjecting the particles to a hydrogen plasma for a time of between 30 minutes and 3 hours at a temperature of between 850 and 1200°C.
4. A method according to any of the preceding claims, in which the support is heated to a temperature of between 100 and 150°C during the spraying and waiting cycle.
5. A method according to any of the preceding claims, in which the spraying and waiting cycle comprises a spraying time of between 10 and 50 ms and a waiting time of between 1 and 3 s.
6. A method according to any of the preceding claims, in which the powder particles have an average grain size of less than or equal to 250 nm.
7. A photocathode produced by a method according to any of the preceding claims.

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