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REVIEW 4 major objections 6 minor 14 references

Vertically stacked amorphous selenium based VUV photodetectors for use in liquid noble detectors

T0 review · 4 major / 6 minor · reviewed 2026-08-12 · deepseek-v4-flash

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Pith's one-line read A vertically stacked aSe photodetector with a graphene top electrode detects light at roughly 130 K, with the graphene increasing the signal about tenfold.

desk verdict A promising device geometry with a title that outruns the data: no VUV light was ever used, so the central VUV claim is extrapolation, not measurement. read the letter →

arxiv 2411.08823 v1 pith:FDYWV4OY submitted 2024-11-13 physics.ins-det hep-ex

Iakovos Tzoka , M. Rooks , A. C. A. Ishida , A. Barajas , V. A. Chirayath , J. Asaadi This is my paper · ORCID
classification physics.ins-dethep-ex PACS 85.60.Gz
keywords amorphousseleniumverticalphotodetectorvacuumultravioletgrapheneelectrodecryogenicdetectorliquidnobledetectorstimeprojectionchamberphotoconductor
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper reports the first vertically stacked amorphous selenium (aSe) photodetector with a graphene top electrode, designed for detecting vacuum ultraviolet (VUV) light in liquid noble detectors. The authors show that the device produces a measurable photoinduced signal at about 130 K when illuminated with a xenon flash lamp, and that adding a graphene layer over the top contact increases the signal amplitude by roughly a factor of ten. The motivation is that aSe can, in principle, convert VUV photons into electron-hole pairs even at cryogenic temperatures, potentially replacing the wavelength-shifting films currently used in liquid argon and xenon time projection chambers. The paper claims that this vertical geometry, with graphene's transparency, gives a more uniform electric field and better charge collection than earlier horizontal designs.

What carries the argument

The central object is the vertically stacked aSe thin-film photoconductor: a bottom metal electrode, a 1 micrometer aSe layer, and a top metal electrode with an open pattern that leaves most of the aSe surface exposed. A multilayer graphene film is transferred over the top contact and the exposed aSe, acting as a transparent electrode that spreads the applied electric field more uniformly across the photoconductor. Absorbed photons create electron-hole pairs in the aSe, and the resulting charges are collected by the electrodes and read out through a charge-sensitive preamplifier. The vertical stack is what makes the electric field perpendicular to the substrate, avoiding the non-uniform fields of earlier interdigitated horizontal designs.

What would settle it

Illuminate the same aSe-graphene device at cryogenic temperature with a vacuum ultraviolet source (for instance, a deuterium lamp or a synchrotron beamline) tuned to 128 nm and 175 nm, and measure whether a photocurrent appears. If the device responds at 200 nm but produces no signal at these shorter wavelengths, the paper's central claim of VUV detection is refuted.

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Extended reading notes

Core claim

On the paper's own terms, the central finding is that a vertically stacked aSe photoconductor with a graphene-including top electrode functions as a light detector at cryogenic temperatures relevant to liquid noble detectors. Under pulsed xenon light from 200 to 1000 nm, the device generated a photoresponse down to about 130 K, and the peak signal rose from roughly 0.5 mV to 45 mV after multilayer graphene was deposited over the top metal contact and the exposed aSe film. The authors attribute this tenfold increase to a more uniform electric field across the aSe layer and improved charge collection. They present this as the first demonstration of a vertical aSe-based VUV photodetector that exploits the wide-band optical transparency of graphene.

Load-bearing premise

The paper assumes that aSe's demonstrated photoresponse to light between 200 and 1000 nm also holds for the vacuum ultraviolet wavelengths of 128 nm and 175 nm that liquid argon and liquid xenon actually emit.

Editorial extensions

If this is right

  • If aSe's sensitivity extends to vacuum ultraviolet wavelengths, liquid argon and xenon detectors could collect scintillation light without wavelength-shifting films, removing their conversion losses and degradation.
  • The vertical geometry with a transparent top electrode is compatible with pixelated anode planes, so a single device could in principle sense both ionization charge and scintillation light.
  • The roughly tenfold signal increase from graphene suggests that further optimization of the transparent electrode could substantially improve charge collection efficiency.
  • Because the device operates at about 130 K, it can be tested directly in liquid noble environments, not only in vacuum.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The paper does not test wavelengths below 200 nm, so the central VUV claim is unverified; a direct follow-up would be repeating the measurement with a 128 nm or 175 nm light source in the actual scintillation bands.
  • If graphene remains transparent at vacuum ultraviolet wavelengths, this vertical stack could also serve as a universal transparent electrode for other cryogenic photoconductors, not just aSe.
  • The opposite zero-bias signal polarities for Ti/Au and Cr contacts indicate that the metal-aSe interface field can be tailored to favor electron or hole collection, suggesting a path to self-biased or bipolar sensors.
  • The temperature-dependent reduction in signal implies that carrier mobility-lifetime products limit performance at 130 K; higher operating voltages, which the authors plan to enable with fused-silica substrates, may partially recover the lost amplitude.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 6 minor

Summary. The paper reports the fabrication and characterization of a vertically stacked amorphous selenium (aSe) photodetector intended for use in liquid noble detectors. The device consists of a 1 μm aSe film between metal contacts on a silicon substrate, with two contact types (Cr and Ti/Au) and, for some devices, a wet-transferred multilayer graphene layer on top. The authors show that illuminating the device with a Xe flash lamp (200–1000 nm) produces photovoltage-like pulses whose amplitude increases with applied bias, depends on the contact metal, and decreases as the temperature is lowered to ~130 K. They further report that adding graphene increases the peak signal amplitude by roughly an order of magnitude. The central claim is that this constitutes the first demonstration of a vertical aSe-based VUV photodetector with a graphene top electrode, and the paper argues that such a device could enable integrated charge and light sensing in future time projection chambers.

Significance. If the reported results are sound, the work is a useful step toward a detector concept that could eventually combine charge and VUV-light sensing in liquid argon or xenon TPCs. The authors are to be credited for demonstrating a vertical aSe geometry, verifying with Raman spectroscopy that the aSe remains amorphous after metal deposition, and showing a measurable photoresponse at ~130 K for a device without graphene. These are concrete, reproducible fabrication and measurement steps. However, the paper's most prominent claim—that the device is a VUV photodetector—is not supported by the presented data, because no VUV light source was used. In addition, the graphene comparison is confounded by the use of different illumination masks. The significance of the work as a device demonstration for 200–1000 nm light is real but modest; the VUV claim, if retained, must be backed by direct measurements or removed.

major comments (4)
  1. [Abstract and §3] The device is never illuminated with VUV light. The Xe flash lamp is specified to emit from 200 nm to 1000 nm (§3), so the shortest photon energy is 6.2 eV. Liquid argon emits at 128 nm (9.7 eV) and liquid xenon at 175 nm (7.1 eV). The abstract's statement that aSe "can efficiently convert VUV light to charges" is supported only by Ref. [6], a DFT study of trigonal selenium, not by a measurement of the fabricated amorphous film at VUV wavelengths. VUV photons have much shorter absorption lengths than 200 nm light, so carrier generation near the top interface (with the metal or graphene contact) may behave differently. The title and abstract therefore overstate what is demonstrated. This is a load-bearing issue because the entire motivation of the detector is VUV sensitivity; it should be fixed either by adding a VUV measurement or by explicitly rewriting the claims to cover the measured 200–1000 nm range and presenting VUV operation as a hypothetical extension.
  2. [§4, Figure 6] The comparison of the device with and without graphene is confounded by the illumination geometry. The text states that "for these measurements we used a smaller mask for illuminating only the region of the device where graphene was grown," and it is not stated that the pre-graphene measurement used the same mask. Since the full mask exposes 12.5 cm² and the smaller mask exposes 4 cm², a change in illuminated area or in which part of the field is active could contribute to the reported ~10× amplitude increase. To support the claim that graphene itself enhances the signal, the authors must compare the same device, same mask, same area, and same bias before and after graphene deposition, with repeated measurements and error bars.
  3. [§4, Figure 5] The cryogenic measurements at ~130 K show a small pulse, but no dark control (lamp-off) traces, no baseline subtraction, and no error bars or repeated-pulse statistics are presented. Because the signal amplitude at low temperature is substantially reduced, it is not clear that the observed feature is photoinduced rather than electrical pickup or thermal baseline drift. The paper should show lamp-off traces at the same temperatures and bias, and report the run-to-run variation, so that the reader can assess whether the small 130 K signal is significant.
  4. [§4, last paragraph; §5] The conclusion attributes the signal increase with graphene to "more uniform electric field and better charge collection," but no field simulation, spatial photoresponse mapping, or electrical measurement of the graphene electrode is provided. Other explanations—such as a change in the active area due to the different mask, a modification of the contact barrier, or doping/interface effects from the graphene transfer—are not excluded. The interpretation should be presented as a hypothesis or supported by additional measurements (e.g., photocurrent mapping with the same mask, or comparison of devices with and without graphene fabricated identically except for the graphene layer).
minor comments (6)
  1. [§2] The phrase "depositing growing multilayer graphene" is confusing; the wet-transfer method deposits rather than grows graphene. Please rephrase.
  2. [§1, Ref. [6]] Ref. [6] is a DFT study of trigonal selenium, but the text uses it to support a claim about amorphous selenium. Please provide a direct experimental reference for aSe VUV sensitivity or explicitly note that this is an extrapolation.
  3. [Throughout] The manuscript contains many spacing and formatting errors (e.g., "with aSecoated", "theaSe"). A careful proofread is needed before submission.
  4. [Figures 4 and 5] The figures lack error bars and clearly defined signal-amplitude measures. Please specify how the peak amplitude is defined (e.g., baseline-to-peak, after preamplifier gain) and include error bars resulting from repeated measurements.
  5. [§3] The optic fiber is described as "VUV-compatible," but the lamp itself has a 200 nm lower cutoff. Please clarify whether any component in the optical path limits the shortest wavelength delivered to the device.
  6. [§4] The temperature-dependent measurements were taken at 25 mHz with one pulse every 40 seconds. It would be helpful to state the temperature uncertainty and whether the temperature was stable during each acquisition.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the reported signals are direct experimental outputs and the interpretive claims are qualitative, so nothing reduces to a fit or to a self-citation by construction.

full rationale

The paper is an experimental characterization rather than a derivation. The reported results (e.g., "the peak amplitude of the photoinduced signal increases with the increase of the applied voltage across the aSe thin film" and "Peak amplitude increased from 0.5 mV to 45 mV approximately an order of 10") are direct oscilloscope/preamp readouts, not quantities obtained by fitting a model and then re-presented as predictions. The explanations of polarity, temperature dependence, and graphene enhancement are qualitative and are not used as inputs to produce the data. Self-citations appear as background: Ref. [6] supports the premise that aSe can convert VUV photons, and Ref. [7] describes the prior horizontal device and the setup reused here; neither is a fitting target, and the present signals are not constrained by those papers' outputs. Therefore no claim reduces to its input by construction. The significant caveat is not circularity: the device is called a VUV photodetector, but the lamp used has a "wide emission spectrum from 200 nm to 1000 nm", so sensitivity at 128 nm (LAr) or 175 nm (LXe) is an extrapolation, not a demonstrated result. That is a validity gap, appropriately flagged as a risk, not a circular step.

Assumptions & free parameters 0 free parameters · 4 assumptions · 0 invented entities

The central claim rests on material properties of aSe and graphene taken from the prior literature, and on the identification of the measured pulses as photoinduced. No free parameters are fitted to data in this work, and no new physical entities are introduced.

assumptions (4)
  • domain assumption aSe efficiently converts VUV photons into electron-hole pairs.
    Stated in the introduction as a known property, supported by references [4,5,6]. The present paper does not measure quantum efficiency at VUV wavelengths.
  • domain assumption Multilayer graphene transferred onto aSe is transparent in the test wavelength range and electrically conductive enough to act as a top electrode.
    Assumed from reference [8] and supplier data [10]; no in-situ measurement of graphene transmission or sheet resistance after transfer is provided.
  • domain assumption The observed pulses are caused by light and are not electrical artifacts.
    Pulses are synchronized with the flash lamp, but no dark control or light-off measurement is shown to exclude noise or pickup.
  • domain assumption The metal-aSe contact creates an internal electric field that sets the zero-bias signal polarity.
    Qualitative work-function argument in Section 4; the contact potential difference is not measured.

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Cite this review

Pith. "Pith review of Vertically stacked amorphous selenium based VUV photodetectors for use in liquid noble detectors." pith.science (2026). https://pith.science/paper/FDYWV4OY

@misc{pith2026241108823,
  author       = {Pith},
  title        = {Pith review of: Vertically stacked amorphous selenium based VUV photodetectors for use in liquid noble detectors},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/FDYWV4OY}},
  note         = {Machine review of arXiv:2411.08823}
}
abstract

We present results from the characterization of a vertically stacked amorphous selenium (aSe)-based photodetector for use in cryogenic environments. aSe has been identified as an ideal photoconductor that can efficiently convert vacuum ultraviolet (VUV) light to charges even at cryogenic temperatures. We have designed and fabricated an aSe device in vertical geometry with top and bottom metal electrodes that produces an electric field perpendicular to the substrate. The top-metal contact has an open design that results in a large fraction of the aSe thin film surface to be active for photodetection. Our experiments show that the vertically stacked aSe device detects light from a Xenon flash lamp in a vacuum environment and can produce measurable signals at \(\sim \)130K. We also demonstrate a significant enhancement in the amplitude of the photoinduced signal by growing graphene on the top-metal contact and the aSe thin film. Our results provide the first demonstration of a vertical aSe based VUV photodetector that utilizes the wide-band optical transparency of graphene top-electrode. Our results could open the doorway to a potentially game-changing solution of an integrated charge and light sensor that can be employed in future large-scale time projection chambers with pixelated anode planes.

Figures

Figures reproduced from arXiv: 2411.08823 by the authors.

Figure 1
Figure 1. aSe device geometry 3. Experimental Setup The experimental setup used to test these devices in vacuum (Figure 2a) is described in [7]. The setup was modified to receive a device in the vertical geometry. The aSe devices are mounted between two PCBs in a sandwich geometry to make contact with the top and bottom metal electrodes. The PCB device holder is mounted on a copper holder such that the substrate (Si wafer) to… view at source ↗
Figure 2
Figure 2. Device testing system 4. Results We used Raman spectroscopy to confirm that the top-metal deposition does not affect the amorphous structure of the underlying selenium. Raman spectrum ( [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Raman spectrum obtained from the aSe device before and after top-metal deposition. The peak at 250 𝑐𝑚−1 is broad and devoid of features seen for m-Se or t-Se [14] pointing to amorphous structure. The peak does not have a change in shape after metal deposition. Before depositing graphene on aSe thin film and the top-metal electrode, the vertical device is tested by illuminating the device with a Xenon flash lamp at d… view at source ↗
Figures from the paper (3 more)
Figure 4
Figure 4. Figure 4: Variation of the photoinduced signal with applied potential Please note that the metal used for making the contact determines the direction of the inherent electric field at the metal-aSe interface. The aSe devices with Ti/Au contact resulted in a negative going signal…
Figure 5
Figure 5. Figure 5: Testing of the vertically stacked aSe device as a function of temperature [PITH_FULL_IMAGE:figures/full_fig_p006_5.png]
Figure 6
Figure 6. Figure 6: Comparison of the same sample with and without graphene. Peak amplitude increased from 0.5 mV to 45mv approximately an order of 10. of graphene is compared with that after graphene deposition in [PITH_FULL_IMAGE:figures/full_fig_p006_6.png]

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Reference graph

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