Tungsten diselenide WSe2

spore: profile
id: tungsten-diselenide
spore_version: 0.2
family: transition-metal-dichalcogenides
name: Tungsten diselenide
abbreviation: WSe2
role: semiconductor and dry lubricant
category: semiconductors
type: gaas
summary: |
  Tungsten diselenide (WSe2, CAS 12067-46-8) is a layered compound of tungsten and selenium with a hexagonal crystal structure. Its layers slide over one another, so sputtered films can work as dry lubricants, and a single layer is a semiconductor with a direct band gap that is studied for transistors. A 2017 study of sputtered WSex films reported friction of 0.04 to 0.07 in both humid air and dry nitrogen. No vacuum, radiation or space test of WSe2 was found in the sources read.
best_for: Dry sliding contacts that must keep friction near 0.04 to 0.07 in humid air as well as dry nitrogen, based on a laboratory film (the film is WSex, not stoichiometric WSe2)
weakest_at: Flight and space evidence (none found). The friction and hardness data come from a selenium-deficient film, and the band gap is reported differently by each source
identity:
  formula: WSe2
  cas: 12067-46-8
  crystal_system: hexagonal (2H), layered, similar to molybdenum disulfide
  space_group: P6_3/mmc (No. 194)
  source: wiki-wse2
  where: 'Infobox: Crystal structure row (hP6, space group P6 3/mmc, No 194), Lattice constant row and Identifiers (CAS Number 12067-46-8)'
tags:
  environments: []
  stresses: []
  applications:
    - mechanisms
    - electronics-and-chips
    - solar-arrays
properties:
  - benchmark: density
    label: Density, tungsten diselenide (Wikipedia infobox)
    value: 9.32
    unit: g/cm3
    scope: material
    conditions:
      basis: solid WSe2, as printed in the infobox
    method: Infobox value as printed (9.32 g/cm3)
    status: ground
    source: wiki-wse2
    where: Infobox, Density row
  - benchmark: density
    label: Density at 20 °C (supplier SDS)
    value: 9.32
    unit: g/cm3
    scope: material
    conditions:
      basis: WSe2 crystal or powder as supplied
      temperature: 20 °C
    method: Section 9 value as printed (density at 20 °C, 9.32 g/cm3)
    status: ground
    source: samat-sds
    where: Section 9, Density at 20 °C (68 °F)
  - benchmark: molar_mass
    label: Molar mass, WSe2
    value: 341.76
    unit: g/mol
    scope: material
    conditions:
      basis: WSe2, molar mass as printed
    method: Infobox value as printed (341.76 g/mol)
    status: ground
    source: wiki-wse2
    where: Infobox, Molar mass row
  - benchmark: melting_point
    label: Melting point (Wikipedia, lower bound)
    value: 1200
    bound: lower
    unit: °C
    scope: material
    conditions:
      basis: printed as more than 1200 °C, so 1200 is a lower bound and not a measured melting point
    method: Infobox value as printed (> 1200 °C)
    status: ground
    source: wiki-wse2
    where: Infobox, Melting point row
  - benchmark: melting_point
    label: Melting point (Ossila SDS, lower bound)
    value: 1200
    bound: lower
    unit: °C
    scope: material
    conditions:
      basis: crystal, printed as >1200 °C in the supplier safety data sheet
    method: Section 9.1 value as printed (>1200 °C)
    status: ground
    source: ossila-sds
    where: Section 9.1, Melting point/freezing point
  - benchmark: band_gap_indirect_bulk
    label: Indirect band gap, bulk (Wikipedia infobox)
    value: 1
    unit: eV
    scope: material
    conditions:
      basis: bulk WSe2, approximate value printed with a tilde
    method: Infobox value as printed (~1 eV, indirect, bulk)
    status: ground
    source: wiki-wse2
    where: Infobox, Band gap row
  - benchmark: band_gap_indirect_bulk
    label: Indirect band gap, bulk (Zhou et al., Nano Letters)
    value: 1.2
    unit: eV
    scope: material
    conditions:
      basis: bulk WSe2 as a p-type semiconductor, approximate value printed with a tilde
    method: Value as printed in the abstract and introduction (~1.2 eV)
    status: ground
    source: zhou2014
    where: Abstract; Introduction
  - benchmark: band_gap_optical_bulk
    label: Optical band gap, bulk (Wikipedia text)
    value: 1.35
    unit: eV
    scope: material
    conditions:
      basis: bulk WSe2; the text does not say whether this gap is direct or indirect
    method: Value as printed in the potential applications section (~1.35 eV)
    status: ground
    source: wiki-wse2
    where: Potential applications section, first paragraph
  - benchmark: band_gap_direct_monolayer
    label: Direct band gap, monolayer (Wikipedia infobox)
    value: 1.7
    unit: eV
    scope: material
    conditions:
      basis: single-layer WSe2, approximate value printed with a tilde
    method: Infobox value as printed (~1.7 eV, direct, monolayer)
    status: ground
    source: wiki-wse2
    where: Infobox, Band gap row
  - benchmark: band_gap_direct_monolayer
    label: Direct band gap, monolayer (Zhou et al., Nano Letters)
    value: 1.65
    unit: eV
    scope: material
    conditions:
      basis: monolayer WSe2, approximate value printed with a tilde
    method: Value as printed in the abstract and introduction (~1.65 eV)
    status: ground
    source: zhou2014
    where: Abstract; Introduction
  - benchmark: lattice_constant_a
    label: Lattice constant a
    value: 0.3297
    unit: nm
    scope: material
    conditions:
      basis: hexagonal WSe2 cell as printed in the infobox (hP6, space group P6 3/mmc)
    method: Infobox value as printed
    status: ground
    source: wiki-wse2
    where: Infobox, Lattice constant row (a = 0.3297 nm)
  - benchmark: lattice_constant_c
    label: Lattice constant c
    value: 1.2982
    unit: nm
    scope: material
    conditions:
      basis: hexagonal WSe2 cell as printed in the infobox (hP6, space group P6 3/mmc)
    method: Infobox value as printed
    status: ground
    source: wiki-wse2
    where: Infobox, Lattice constant row (c = 1.2982 nm)
  - benchmark: elastic_modulus
    label: Young's modulus, single monolayer (Wikipedia text)
    value: 258.6
    uncertainty: ± 38.3 GPa
    unit: GPa
    scope: material
    conditions:
      basis: single WSe2 monolayer; the test method is not stated in the Wikipedia text
    method: Value as printed in the Wikipedia article, which cites a primary paper that was not read in this run
    status: ground
    source: wiki-wse2
    where: Structure and properties section, paragraph on the number of layers in a flake
  - benchmark: hole_mobility
    label: Hole mobility, monolayer (up to)
    value: 100
    bound: upper
    unit: cm2 V-1 s-1
    scope: material
    conditions:
      basis: p-type monolayer WSe2 field-effect transistor from large-area CVD growth
      temperature: room temperature
    method: Value as printed (up to 100 cm2 V-1 s-1 for monolayer at room temperature; printed with spaces, written here without)
    status: ground
    source: zhou2014
    where: Abstract; Results (field-effect transistor section)
  - benchmark: hole_mobility
    label: Hole mobility, few-layer (up to)
    value: 350
    bound: upper
    unit: cm2 V-1 s-1
    scope: material
    conditions:
      basis: few-layer WSe2 films from the same large-area CVD growth
      temperature: room temperature
    method: Value as printed (up to 350 cm2 V-1 s-1 for few-layer material; printed with spaces, written here without)
    status: ground
    source: zhou2014
    where: Abstract; Results; Conclusion
  - benchmark: coefficient_of_friction
    label: Friction, sputtered WSex film in humid air and dry nitrogen
    value: 0.04
    value_high: 0.07
    bound: range
    unit: '-'
    scope: material
    conditions:
      basis: pure WSex film (selenium to tungsten ratio below the stoichiometric 2) by RF magnetron sputtering
      air: ambient air, 30 to 40 percent RH
      nitrogen: dry nitrogen, below 7 percent RH
      test: reciprocating sliding, same tribometer for all coatings
    method: Range as printed in the text (0.04 to 0.07, with an en dash); same value in both atmospheres
    status: ground
    source: wsex2017
    where: Results and discussion, paragraph beginning 'The tribological behavior remains almost unaltered'
  - benchmark: coefficient_of_friction
    label: Steady-state friction, sputtered WSex film in dry nitrogen, long test
    value: 0.03
    value_high: 0.05
    bound: range
    unit: '-'
    scope: material
    conditions:
      basis: WSex film in dry nitrogen below 7 percent RH, rotating ball-on-disk tribometer
      cycles: 125000
      load: 2 N
      sliding_speed: 5 cm s-1
    method: Range as printed in the text for the WSex film (0.03 to 0.05, with an en dash)
    status: ground
    source: wsex2017
    where: Results, long-duration tests in dry nitrogen (paragraph ending with the friction curves, Figure 12)
  - benchmark: hardness
    label: Hardness, sputtered WSex film
    value: 5.1
    unit: GPa
    scope: material
    conditions:
      basis: WSex film, about 0.9 μm thick, Vickers microindentation at loads up to 10 mN
    method: Table 1, Hardness (GPa) column, WSex row
    status: ground
    source: wsex2017
    where: Table 1, WSex row
  - benchmark: elastic_modulus
    label: Elastic modulus, sputtered WSex film
    value: 90
    unit: GPa
    scope: material
    conditions:
      basis: WSex film, Vickers microindentation at loads up to 10 mN
    method: Table 1, Elastic modulus (GPa) column, WSex row
    status: ground
    source: wsex2017
    where: Table 1, WSex row
corrosion:
  - environment: Ambient air (30 to 40 percent RH) against dry nitrogen (below 7 percent RH), sputtered WSex film
    headline: The WSex film kept the same friction in humid air and in dry nitrogen.
    effect: Friction stayed in the same 0.04 to 0.07 band in both atmospheres. The authors state that the selenides are less moisture sensitive than MoS2, whose friction rose in humid air. They attribute part of the stable WSex behaviour to W-rich areas that capture oxygen, form tungsten oxides and keep the WSe2 phase.
    rating: low
    conditions:
      basis: RF magnetron sputtered WSex film (selenium to tungsten ratio below 2), reciprocating sliding, same tribometer for all coatings
      humidity_air: 30 to 40 percent RH
      humidity_nitrogen: below 7 percent RH
    source: wsex2017
    where: Results and discussion, paragraph beginning 'The tribological behavior remains almost unaltered'; the paragraph on moisture sensitivity; Discussion of W-rich areas
  - environment: Free surface of WSe2 heated or exposed to air
    headline: No temperature at which WSe2 oxidises in air was found in the sources read.
    effect: The Ossila safety data sheet lists the decomposition temperature as not known, and the SA Materials sheet gives no melting point. Neither source reports an oxidation test. A secondhand onset temperature in a search snippet was not used because the cited paper was not read in this run.
    rating: unknown
    conditions:
      basis: no temperature, time or atmosphere given in the sources read
    source: ossila-sds
    where: Section 9.1, Decomposition temperature (Not known); Section 9.1, Melting point
risks:
  hazards:
    - text: 'The Ossila safety data sheet classes tungsten diselenide as Danger: toxic if swallowed or if inhaled (H301 and H331), may cause damage to organs through prolonged or repeated exposure (H373), and is very toxic to aquatic life with long lasting effects (H400 and H410).'
      ghs:
        - H301
        - H331
        - H373
        - H400
        - H410
      source: ossila-sds
      where: Section 2, Hazard statement(s) and signal word
    - text: The Stanford Advanced Materials safety data sheet gives the Danger signal word with H301 and H331 (toxic if swallowed or if inhaled) and H373 (damage to the central nervous system, liver and digestive system through prolonged or repeated exposure).
      ghs:
        - H301
        - H331
        - H373
      source: samat-sds
      where: Section 2, Classification and Hazard statements
    - text: The Wikipedia infobox gives GHS06, GHS08 and GHS09 pictograms with the Warning signal word and the hazard statements H301, H331, H373 and H410. It links an external MSDS from a metals supplier, which was not read in this run.
      ghs:
        - H301
        - H331
        - H373
        - H410
      source: wiki-wse2
      where: Infobox, Occupational safety and health row (GHS labelling)
    - text: PubChem's aggregated ECHA notification summary for tungsten selenide gives the Danger signal word for H301 and H331, and lists H373, H400 and H410 with the Warning word.
      ghs:
        - H301
        - H331
        - H373
        - H400
        - H410
      source: pubchem-wse2
      where: GHS Classification section (aggregated ECHA C&L notifications)
  failure_risks:
    - mode: Debris and wear in sliding contact in ambient air
      trigger: Sliding in humid ambient air, compared with dry nitrogen
      consequence: The 2017 study reports more debris and more material adhered to the counter-ball in ambient air than in dry nitrogen, and attributes this to water and oxygen causing oxidation and film wear.
      severity: low
      source: wsex2017
      where: Discussion of counterface micrographs (Figure 9) and the paragraph that follows the WSex results
    - mode: Lubrication data that describe a selenium-deficient film
      trigger: Sputter deposition under the 2017 study conditions
      consequence: The film's selenium to tungsten ratio did not reach the stoichiometric value of 2, so its friction, hardness and modulus describe a selenium-deficient WSex film. A stoichiometric WSe2 film may behave differently, and that was not tested in the sources read.
      severity: moderate
      source: wsex2017
      where: Table 1 and the chemical composition paragraph (EPMA results)
  side_effects:
    - text: In a fire the Ossila sheet lists tungsten oxides and selenium or selenium oxides as special hazards.
      affects: Firefighters and people nearby
      source: ossila-sds
      where: Section 5.2, Special hazards arising from the substance or mixture
    - text: The SA Materials sheet lists hydrogen selenide and metal oxide fume as products that can be released in a fire.
      affects: Firefighters and people nearby
      source: samat-sds
      where: Section 5, Special hazards arising from the substance or mixture
  handling:
    - text: Keep the container tightly closed, store it in a well-ventilated place and locked up, at ambient temperature (Ossila SDS, sections 7.2 and 9.1).
      source: ossila-sds
      where: Section 7.2, Conditions for safe storage; Section 9.1, Storage temperature
    - text: Keep the container tightly sealed in a cool, dry place with good ventilation at the workplace (SA Materials SDS).
      source: samat-sds
      where: Section 7, Precautions for safe handling and storage
    - text: Avoid breathing dust (precautionary statement P261) and avoid contact with skin and eyes.
      source: ossila-sds
      where: Section 2, Precautionary statement(s); Section 7.1, Advice on safe handling
  mitigations:
    - text: Provide adequate ventilation and exhaust, with a washing facility for eyes and skin, and wear eye protection (Ossila SDS).
      source: ossila-sds
      where: Section 8.2, Appropriate engineering controls and personal protection
uses:
  - application: mechanisms
    device: Sputtered WSex solid-lubricant film for vacuum and space contacts (proposed in the 2017 study)
    example: The 2017 study concludes that the WSex nanocomposite is a good competitor for MoS2 as a solid lubricant for vacuum and space applications where components see variable surroundings. The films were tested in laboratory tribometers, not in space.
    source: wsex2017
    where: Conclusions, last paragraph
  - application: electronics-and-chips
    device: p-type monolayer transistor channel (laboratory device)
    example: Zhou et al. report p-type WSe2 field-effect transistors with excellent electronic characteristics, with hole mobility of up to 100 cm2 V-1 s-1 for a monolayer and up to 350 cm2 V-1 s-1 for few-layer films, both at room temperature.
    source: zhou2014
    where: Abstract; Results (field-effect transistor section)
  - application: solar-arrays
    device: Exfoliated monolayer film for photovoltaic and LED research (laboratory, not flight)
    example: Wikipedia describes mechanically exfoliated WSe2 monolayers as transparent photovoltaic materials with LED properties, and notes that the material can be switched from p-type to n-type by changing the voltage of an adjacent metal electrode.
    source: wiki-wse2
    where: Potential applications section
supply:
  - subject: Tungsten (feedstock for WSe2)
    producers:
      - country: China
        tonnes: 67000
        share_percent: 78.8
        year: 2025
      - country: Vietnam
        tonnes: 3000
        share_percent: 3.5
        year: 2025
      - country: Kazakhstan
        tonnes: 2400
        share_percent: 2.8
        year: 2025
      - country: Russia
        tonnes: 2000
        share_percent: 2.4
        year: 2025
    critical_lists:
      - list: EU critical raw materials list (Regulation (EU) 2024/1252, as shown on the European Commission page)
        listed: true
    recycling: 'USGS 2026: the amount of secondary tungsten produced and consumed in 2025 was withheld to avoid disclosing company data.'
    substitutes:
      - USGS 2026 lists molybdenum, carbides based on molybdenum, niobium or titanium, and tool steels as potential substitutes for some tungsten uses; it does not discuss tungsten diselenide.
    source: usgs-w-2026
    where: World Mine Production and Reserves table, 2025 column (world total 85,000 t, rounded); Events, Trends and Issues; Recycling; Substitutes. Shares calculated by Space Mushroom as tonnes divided by the printed world total. Critical-list status checked against the European Commission page (source key ec-crm).
  - subject: Selenium (byproduct of copper refining, feedstock for WSe2)
    producers:
      - country: China
        tonnes: 2000
        share_percent: 53
        year: 2025
      - country: Japan
        tonnes: 640
        share_percent: 16.8
        year: 2025
      - country: Russia
        tonnes: 320
        share_percent: 8.4
        year: 2025
      - country: Belgium
        tonnes: 200
        share_percent: 5.3
        year: 2025
    critical_lists:
      - list: EU critical raw materials list (Regulation (EU) 2024/1252, as shown on the European Commission page)
        listed: false
    recycling: 'USGS 2026: selenium recycling is insignificant, and most scrap from electronic materials was exported for recovery of contained selenium.'
    substitutes:
      - USGS 2026 names amorphous silicon and cadmium telluride as the two principal competitors of CIGS thin-film solar cells.
    source: usgs-se-2026
    where: World Refinery Production table, 2025 column (world total 3,800 t); Events, Trends and Issues (China 53 percent of estimated global production). Shares for countries other than China calculated by Space Mushroom as tonnes divided by 3,800 t.
documents:
  - title: 'Safety Data Sheet: Tungsten Diselenide Crystal (Ossila Limited)'
    type: sds
    url: https://downloads.ossila.com/msds/tungsten-diselenide-crystal.pdf
    year: 2026
  - title: 'Safety Data Sheet: Tungsten Diselenide Crystal (Stanford Advanced Materials)'
    type: sds
    url: https://www.samaterials.com/tds/1697121360-tungsten-diselenide-wse2-crystal-sds.pdf
    year: 2023
  - title: Large Area Growth and Electrical Properties of p-Type WSe2 Atomic Layers (Nano Letters)
    type: paper
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC4296926
    year: 2015
  - title: Solid lubricant behavior of MoS2 and WSe2-based nanocomposite coatings (Science and Technology of Advanced Materials)
    type: paper
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC5402748/
    year: 2017
  - title: 'PubChem: Tungsten selenide (WSe2), CID 82910, GHS classification'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/82910
    year: 2026
  - title: 'USGS Mineral Commodity Summaries 2026: Tungsten'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-tungsten.pdf
    year: 2026
  - title: 'USGS Mineral Commodity Summaries 2026: Selenium'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-selenium.pdf
    year: 2026
gaps:
  - No vacuum, thermal-vacuum, radiation, low-Earth-orbit or atomic-oxygen result for WSe2 was found in the sources read. The 2017 study calls WSex a good competitor for MoS2 in vacuum and space; that is its conclusion, not a test.
  - The 2017 film is substoichiometric (selenium to tungsten below the stoichiometric 2), so its friction, hardness and modulus rows describe WSex, not stoichiometric WSe2. No friction or hardness value for stoichiometric WSe2 was found in the sources read.
  - 'Melting point: Wikipedia and the Ossila SDS print more than 1200 °C, while the SA Materials SDS prints no data. The 1200 °C value is recorded as a lower bound.'
  - 'Bulk band gap disagrees across sources: about 1 eV (Wikipedia infobox, indirect), about 1.2 eV (Zhou et al., indirect) and about 1.35 eV (Wikipedia text, optical). All three are kept with their sources and none is reconciled.'
  - 'Monolayer direct band gap: about 1.7 eV in the Wikipedia infobox and about 1.65 eV in Zhou et al. Both are kept with their sources.'
  - The oxidation onset temperature of WSe2 in air was not found in any source read. Wear rates in the 2017 study are printed in exponent notation and were left out of the record.
  - The 2017 text gives a humid-air friction range for tungsten selenide coatings in general. It was not recorded because it refers to the coating family rather than the WSex film measured in the paper's tables.
  - Lorad Chemical's catalogue listing (tungsten diselenide, CAS 12067-46-8) shows density and melting point in line with Wikipedia, but the page blocked automated access in this run, so it is not cited for any value.
  - The USGS 2025 Critical Minerals list was not checked because the USGS page blocked access in this run. Tungsten and selenium are shown only against the EU critical raw materials list.
  - No UNS, EN, ASTM or other grade designations apply to WSe2, which is a chemical compound rather than a metal grade, so the designations list is empty. The CAS number is in identity.
  - 'Benchmark names not in spores/property-canon.yaml and used here: molar_mass, band_gap_indirect_bulk, band_gap_optical_bulk, band_gap_direct_monolayer, lattice_constant_a, lattice_constant_c, hole_mobility, coefficient_of_friction and hardness. Check them against the canon before import.'
  - Hazard data rest on two supplier SDSs (which differ in signal word and melting point data), the Wikipedia infobox and PubChem's aggregated ECHA summary. No eye or skin irritation statement for WSe2 was found.
  - Film-level values (friction, hardness, modulus) are from a single research study, and the Wikipedia monolayer modulus is not traced to its primary paper in this run.
story:
  recipe:
    line: 'WSe2 is made of one tungsten atom and two selenium atoms per formula unit. Wikipedia gives the synthesis as W + 2 Se giving WSe2: tungsten films are sputter-deposited under gaseous selenium at high temperature, which yields films that crystallise in a hexagonal structure with the correct stoichiometric ratio.'
    parts:
      - name: Tungsten
        symbol: W
        atoms: 1
        atomic_weight: 183.84
        role: The metal centre of each formula unit. It sits between two selenium layers in trigonal prismatic coordination.
        source: nist-w
        where: Standard atomic weight column for tungsten
      - name: Selenium
        symbol: Se
        atoms: 2
        atomic_weight: 78.971
        role: Forms the top and bottom layers of each sheet. Weak bonds between the sheets let them slide, which is the basis of the lubricant behaviour.
        source: nist-se
        where: Standard atomic weight column for selenium
  risk_groups:
    - title: For people
      level: 2
      verdict: Toxic if swallowed or inhaled, per SDS
      lead: The supplier safety data sheets classify tungsten diselenide as Danger for acute oral and inhalation toxicity and for organ damage from repeated exposure. PubChem's aggregated summary gives the same toxicity codes.
      points:
        - label: Toxicity
          line: H301 and H331 (toxic if swallowed or if inhaled) appear in both supplier sheets, in the Wikipedia infobox and in PubChem's aggregated summary.
        - label: Organs
          line: H373 (damage to organs through prolonged or repeated exposure) appears in both supplier sheets and in the Wikipedia infobox.
        - label: Aquatic life
          line: H400 and H410 (very toxic to aquatic life) appear in the Ossila sheet and in PubChem's summary. The Wikipedia infobox lists H410 only.
    - title: Dust and fire
      verdict: Avoid dust. Fire gives off selenium oxides
      lead: The powder can be inhaled, and a fire releases toxic oxides and selenium compounds.
      points:
        - label: Dust
          line: Avoid breathing dust (P261 in the Ossila sheet) and provide exhaust ventilation where dust forms.
        - label: In a fire
          line: The Ossila sheet lists tungsten oxides and selenium oxides. The SA Materials sheet lists hydrogen selenide and metal oxide fume.
    - title: Using the film
      verdict: Humid air is tolerated in lab tests
      lead: Lab results from the 2017 study suggest the film keeps its friction in humid air, but the film was selenium-deficient and no air-oxidation temperature was found.
      points:
        - label: Humidity
          line: The WSex film kept its friction in humid air and in dry nitrogen. The study says water and oxygen in the contact still increase debris and wear for the coatings it tested in ambient air.
        - label: Oxidation temperature
          line: No temperature at which WSe2 oxidises in air was found in the sources read.
  why: 'Tungsten diselenide (WSe2, CAS 12067-46-8) is a layered compound of tungsten and selenium. Each layer has tungsten sitting between two selenium sheets, and the layers are stacked loosely, which lets them slide. The same layered structure serves two uses: as a dry lubricant when sputtered into a coating, and as a semiconductor when made into single or few atomic layers. Bulk WSe2 is an indirect-gap semiconductor and a single layer has a direct gap, which is why it is studied for transistors and photovoltaics.'
  uses:
    - where: Dry lubricant coatings (laboratory)
      line: A 2017 study of sputtered WSex films concluded that the nanocomposite is a good competitor for MoS2 as a solid lubricant for vacuum and space applications where components see variable surroundings. The films were tested in laboratory tribometers, not in space.
    - where: Transistor channels (laboratory)
      line: Zhou et al. built p-type WSe2 field-effect transistors from large-area films. Hole mobility was up to 100 cm2 V-1 s-1 for a monolayer and up to 350 cm2 V-1 s-1 for few-layer films, both at room temperature.
    - where: Photovoltaic and LED films (laboratory)
      line: Wikipedia describes mechanically exfoliated WSe2 monolayers as transparent photovoltaic materials with LED properties. These are research results, not flight hardware.
  can_do:
    - label: Density
      benchmark: density
      line: Solid WSe2 has a density of 9.32 g/cm3 in the Wikipedia infobox, and the SA Materials sheet gives the same value at 20 °C.
    - label: Melting point
      benchmark: melting_point
      line: The melting point is printed as more than 1200 °C in Wikipedia and in the Ossila sheet. The SA Materials sheet gives no melting point.
    - label: Band gap
      benchmark: band_gap_direct_monolayer
      line: A single layer has a direct band gap of about 1.7 eV (Wikipedia) or about 1.65 eV (Zhou et al.). Bulk WSe2 has an indirect gap of about 1 eV (Wikipedia) or about 1.2 eV (Zhou et al.).
    - label: Friction
      benchmark: coefficient_of_friction
      line: A sputtered WSex film gave friction of 0.04 to 0.07 in ambient air at 30 to 40 percent RH and in dry nitrogen below 7 percent RH. In a long test in dry nitrogen it reached 0.03 to 0.05.
    - label: Hardness and stiffness
      benchmark: hardness
      line: The sputtered WSex film had a hardness of 5.1 GPa and an elastic modulus of 90 GPa (Table 1 of the 2017 study).
  compare: 'Within the layered solid lubricants tested in the 2017 study, the sputtered WSex film was the more moisture-tolerant one. Its friction stayed at 0.04 to 0.07 in both humid air and dry nitrogen, while the MoS2 film from the same study rose in humid air. The authors state that the selenides are less moisture sensitive than MoS2. The bulk band gap is not settled: the Wikipedia infobox gives about 1 eV, Zhou et al. give about 1.2 eV, and the Wikipedia text gives an optical band gap of about 1.35 eV. Quote the value together with its source.'
  compare_rows:
    - label: Density
      benchmark: density
    - label: Melting point
      benchmark: melting_point
    - label: Friction in humid air and dry nitrogen
      benchmark: coefficient_of_friction
  handling:
    - The powder is classed as toxic by its supplier safety data sheets. Avoid breathing dust and use exhaust ventilation where dust forms.
    - Keep it in a tightly closed container in a dry, well-ventilated place, and store it locked up.
    - The Ossila sheet lists no known incompatible materials, and the SA Materials sheet reports no dangerous reactions known.
  good_for:
    - 'Dry sliding where humidity is a concern: the 2017 WSex film held friction at 0.04 to 0.07 in both humid air and dry nitrogen (laboratory result).'
    - 'Low friction in dry nitrogen: the same film reached 0.03 to 0.05 in its long test.'
    - 'Research on layered semiconductors: laboratory monolayer devices reached hole mobility of up to 100 cm2 V-1 s-1 at room temperature.'
  watch_out:
    - The lubricant data come from a selenium-deficient film (selenium to tungsten below the stoichiometric 2), not from stoichiometric WSe2.
    - The band gap and melting point differ between sources (see the properties section), so quote them with their sources.
    - No vacuum, radiation or space test of WSe2 was found in the sources read.
    - 'The hazard signal word differs: the Wikipedia infobox and PubChem''s summary use Warning for some codes, while the Ossila and SA Materials sheets use Danger.'
  verdict: WSe2 is a credible layered semiconductor and a humidity-tolerant dry-lubricant candidate in laboratory tests. Its space evidence is thin, and its toxicity classification means the powder needs controlled handling.
updated: '2026-11-01'
review:
  status: draft
  method: assistant
  checked_values: 19
  unreachable: 2
  caveats: 4
  note: 'Written 2026-11-01 by an AI assistant from pages read in this run: Wikipedia (tungsten diselenide), Zhou et al. (PMC4296926), the 2017 WSex and MoS2 study (PMC5402748), the Ossila and Stanford Advanced Materials safety data sheets, PubChem CID 82910, USGS Mineral Commodity Summaries 2026 for tungsten and selenium, the European Commission critical raw materials page and NIST atomic weights. Type: none of the semiconductors types in taxonomy.ts fits a layered dichalcogenide, so the nearest entry is used; review the type before publication.'
sources:
  wiki-wse2:
    title: Tungsten diselenide
    id: Wikipedia, English edition
    url: https://en.wikipedia.org/wiki/Tungsten_diselenide
    kind: web
    license: CC BY-SA 4.0
  zhou2014:
    title: Large Area Growth and Electrical Properties of p-Type WSe2 Atomic Layers
    id: doi:10.1021/nl504256y (Nano Letters 15, 709-713, PMC4296926)
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC4296926
    year: 2015
    kind: paper
  wsex2017:
    title: Solid lubricant behavior of MoS2 and WSe2-based nanocomposite coatings
    id: Dominguez-Meister et al., Science and Technology of Advanced Materials, 2017, doi:10.1080/14686996.2016.1275784 (PMC5402748)
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC5402748/
    year: 2017
    kind: paper
    license: open access (PMC)
  ossila-sds:
    title: 'Safety Data Sheet: Tungsten Diselenide Crystal (Ossila Limited, revision 6)'
    id: Ossila SDS, last review 03/09/2026, CAS 12067-46-8
    url: https://downloads.ossila.com/msds/tungsten-diselenide-crystal.pdf
    year: 2026
    kind: datasheet
    license: supplier document, linked not reproduced
  samat-sds:
    title: 'Safety Data Sheet: Tungsten Diselenide Crystal (Stanford Advanced Materials)'
    id: SDS revised 23/01/2023, CAS 12067-46-8
    url: https://www.samaterials.com/tds/1697121360-tungsten-diselenide-wse2-crystal-sds.pdf
    year: 2023
    kind: datasheet
    license: supplier document, linked not reproduced
  pubchem-wse2:
    title: PubChem compound record, Tungsten selenide (WSe2), CID 82910
    id: CID 82910; CAS 12067-46-8
    url: https://pubchem.ncbi.nlm.nih.gov/compound/82910
    year: 2026
    kind: database
    license: public domain (NLM)
  usgs-w-2026:
    title: Mineral Commodity Summaries 2026, Tungsten
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-tungsten.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  usgs-se-2026:
    title: Mineral Commodity Summaries 2026, Selenium
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-selenium.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  ec-crm:
    title: Critical raw materials (European Commission, list under Regulation (EU) 2024/1252)
    id: Regulation (EU) 2024/1252
    url: https://single-market-economy.ec.europa.eu/sectors/raw-materials/areas-specific-interest/critical-raw-materials_en
    year: 2024
    kind: web
    license: European Commission web page
  nist-w:
    title: Atomic weight of tungsten (NIST, Atomic Weights and Isotopic Compositions)
    id: NIST Physical Measurement Laboratory
    url: https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=W&all=all
    kind: database
    license: public domain (NIST)
  nist-se:
    title: Atomic weight of selenium (NIST, Atomic Weights and Isotopic Compositions)
    id: NIST Physical Measurement Laboratory
    url: https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=Se&all=all
    kind: database
    license: public domain (NIST)
license: CC-BY-4.0